Mechanisms for reduced density CSI-RS

ABSTRACT

According to some embodiments, a method for use in a network node of transmitting channel slate information reference signals (CSI-RS) comprises: transmitting, to the wireless device, an indication of the subset of PRBs that the wireless device should use to measure CSI-RS; and transmitting CSI-RS on the indicated subset of PRBs. According to some embodiments, a method for use in a wireless device of receiving CSI-RS comprises: receiving an indication of a subset of PRBs that the wireless device should use to measure CSI-RS associated with an antenna port; and receiving CSI-RS on the indicated subset of PRBs. In some embodiments, the indication of the subset of PRBs that the wireless device should use to measure CSI-RS comprises a density value and a comb offset.

PRIORITY

This application is a continuation, under 35 U.S.C. § 120 of U.S. patentapplication Ser. No. 16/301,219 filed on Nov. 13, 2018 which is a U.S.National Stage Filing under 35 U.S.C. § 371 of International PatentApplication Serial No. PCT/SE2017/050502 filed May 15, 2017 and entitled“Mechanisms for Reduced Density CSI-RS” which claims priority to U.S.Provisional Patent Application No. 62/335,989 filed May 13, 2016 both ofwhich are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

Particular embodiments are directed to wireless communications and, moreparticularly, to mechanisms for reduced density channel stateinformation reference signal (CSI-RS).

BACKGROUND

Third Generation Partnership Project (3GPP) Long Term Evolution (LTE)uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink,where each downlink symbol may be referred to as an OFDM symbol, andDiscrete Fourier Transform (DFT)-spread OFDM in the uplink, where eachuplink symbol may be referred to as an SC-FDMA symbol. The basic LTEdownlink physical resource comprises a time-frequency grid asillustrated in FIG. 1 .

The next generation mobile wireless communication system (5G or NR),supports a diverse set of use cases and a diverse set of deploymentscenarios. The later includes deployment at both low frequencies (100sof MHz), similar to LTE today, and very high frequencies (mm waves inthe tens of GHz). At high frequencies, propagation characteristics makeachieving good coverage challenging. One solution to the coverage issueis to employ high-gain beamforming, typically in an analog manner, toachieve satisfactory link budget. Beamforming may also be used at lowerfrequencies (typically digital beamforming), and is expected to besimilar in nature to the already standardized 3GPP LTE system (4G).

FIG. 1 illustrates an example downlink radio subframe. The horizontalaxis represents time and the other axis represents frequency. Radiosubframe 10 includes resource elements 12. Each resource element 12corresponds to one OFDM subcarrier during one OFDM symbol interval. Inthe time domain, LTE downlink transmissions may be organized into radioframes.

LTE and NR use OFDM in the downlink and DFT-spread OFDM or OFDM in theuplink. The basic LTE or NR downlink physical resource can thus be seenas a time-frequency grid as illustrated in FIG. 1 , where each resourceelement corresponds to one OFDM subcarrier during one OFDM symbolinterval. Although a subcarrier spacing of Δf=15 kHz is shown in FIG. 1, different subcarrier spacing values are supported in NR. The supportedsubcarrier spacing values (also reference to as different numerologies)in NR are given by Δf=(15×2^(α)) kHz where α is a non-negative integer.

FIG. 2 illustrates an example radio frame. Radio frame 14 includessubframes 10. In LTE, each radio frame 14 is 10 ms and consists of tenequally-sized subframes 10 of length Tsubframe=1 ms. In LTE, for normalcyclic prefix, one subframe consists of 14 OFDM symbols and the durationof each symbol is approximately 71.4 μs. In NR, subframe length is fixedat 1 ms regardless of the numerology used. In NR, the slot duration fora numerology of (15×2^(α)) kHz is given by ½^(α) ms assuming 14 OFDMsymbols per slot, and the number of slots per subframe depends on thenumerology.

Users are allocated a specific number of subcarriers for a predeterminedamount of time. These are referred to as physical resource blocks(PRBs). PRBs thus have both a time and frequency dimension. In LTE, aresource block corresponds to one slot (0.5 ms) in the time domain and12 contiguous subcarriers in the frequency domain. Resource blocks arenumbered in the frequency domain, starting with 0 from one end of thesystem bandwidth. For NR, a resource block is also 12 subcarriers infrequency but may span one or more slots in the time domain.

Downlink transmissions are dynamically scheduled, i.e., in each subframethe base station transmits control information about to which terminalsdata is transmitted and upon which resource blocks the data istransmitted, in the current downlink subframe. In LTE, the controlsignaling is typically transmitted in the first 1, 2, 3 or 4 OFDMsymbols in each subframe.

FIG. 3 illustrates an example downlink subframe. Subframe 10 includesreference symbols and control signaling. In the illustrated example, thecontrol region includes 3 OFDM symbols. The reference symbols includecell specific reference symbols (CRS) which may support multiplefunctions including fine time and frequency synchronization and channelestimation for certain transmission modes.

LTE includes a number of physical downlink channels. A downlink physicalchannel corresponds to a set of resource elements carrying informationoriginating from higher layers. The following are some of the physicalchannels supported in LTE: Physical Downlink Shared Channel (PDSCH);Physical Downlink Control Channel (PDCCH); Enhanced Physical DownlinkControl Channel (EPDCCH); Physical Uplink Shared Channel (PUSCH); andPhysical Uplink Control Channel (PUCCH).

PDSCH is used mainly for carrying user traffic data and higher layermessages. PDSCH is transmitted in a downlink subframe outside of thecontrol region as shown in FIG. 3 . Both PDCCH and EPDCCH are used tocarry Downlink Control Information (DCI) such as PRB allocation,modulation level and coding scheme (MCS), precoder used at thetransmitter, etc. PDCCH is transmitted in the first one to four OFDMsymbols in a downlink subframe (i.e., the control region), while EPDCCHis transmitted in the same region as PDSCH.

LTE defines different DCI formats for downlink and uplink datascheduling. For example, DCI formats 0 and 4 are used for uplink datascheduling while DCI formats 1, 1A, 1B, 1C, 1D, 2, 2A, 2B, 2C, 2D, 3/3Aare used for downlink data scheduling. Two search spaces are defined forPDCCH (i.e., a common search space and a UE specific search space).

The common search space consists of PDCCH resources over which all userequipment (UEs) monitor for PDCCH(s). A PDCCH intended for all or agroup of UEs is always transmitted in the common search space so all UEscan receive it.

The UE specific search space consists of PDCCH resources that can varyfrom UE to UE. A UE monitors both the common search space and the UEspecific search space associated with it for PDCCH(s). DCI 1C carriesinformation for PDSCH intended for all UEs or for UEs that have not beenassigned with a Radio Network Temporary Identifier (RNTI), so it isalways transmitted in the common search space. DCI 0 and DCI 1A can betransmitted on either common or UE specific search space. DCI 1B, 1D, 2,2A, 2C and 2D are always transmitted on UE specific search space.

In downlink, which DCI format is used for data scheduling is associatedwith a downlink transmission scheme and/or the type of message to betransmitted. The following are some of the transmission schemessupported in LTE: single-antenna port; transmit diversity (TxD);open-loop spatial multiplexing; closed-loop spatial multiplexing; and upto 8 layer transmission.

PDCCH is always transmitted with either the single-antenna port or TxDscheme while PDSCH can use any one of the transmission schemes. In LTE,a UE is configured with a transmission mode (TM), rather than atransmission scheme. There are 10 TMs (i.e., TM1 to TM10) defined forPDSCH in LTE. Each TM defines a primary transmission scheme and a backuptransmission scheme. The backup transmission scheme is either singleantenna port or TxD. The primary transmission schemes in LTE include:TM1: single antenna port, port 0; TM2: TxD; TM3:open-loop SM; TM4:closed-loop SM; TM9: up to 8 layer transmission, port 7-14; and TM10: upto 8 layer transmission, port 7-14.

In TM1 to TM6, cell specific reference signal (CRS) is used as thereference signal for both channel state information feedback and fordemodulation at a UE. In TM7 to TM10, UE specific demodulation referencesignal (DMRS) is used as the reference signal for demodulation.

LTE includes codebook-based precoding. Multi-antenna techniques cansignificantly increase the data rates and reliability of a wirelesscommunication system. The performance is in particular improved if boththe transmitter and the receiver are equipped with multiple antennas,which results in a multiple-input multiple-output (MIMO) communicationchannel. Such systems and/or related techniques are commonly referred toas MIMO.

A core component in LTE is the support of MIMO antenna deployments andMIMO related techniques. Currently, up to 8-layer spatial multiplexingwith 2, 4, 8, 16 1D transmit (Tx) antenna ports and 8, 12, and 16 Tx 2Dantenna ports are supported in LTE with channel dependent precoding. Thespatial multiplexing mode is aimed for high data rates in favorablechannel conditions. FIG. 4 illustrates example spatial multiplexingoperation.

FIG. 4 is a block diagram illustrating the logical structure of precodedspatial multiplexing mode in LTE. The information carrying symbol vectors is multiplied by an N_(T)×r precoder matrix W, which serves todistribute the transmit energy in a subspace of the N_(T) (correspondingto N_(T) antenna ports) dimensional vector space.

The precoder matrix is typically selected from a codebook of possibleprecoder matrices, and is typically indicated by a precoder matrixindicator (PMI), which specifies a unique precoder matrix in thecodebook for a given number of symbol streams. The r symbols in s eachcorrespond to a layer and r is referred to as the transmission rank.Spatial multiplexing is achieved because multiple symbols can betransmitted simultaneously over the same time/frequency resource element(TFRE). The number of symbols r is typically adapted to suit the currentchannel properties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink). Thereceived N_(R)×1 vector y_(n) for a certain TFRE on subcarrier n (oralternatively data TFRE number n) is thus modeled by)y _(n) =H _(n) Ws _(n) +e _(n)  Equation 1where e_(n) is a noise/interference vector. The precoder W can be awideband precoder, which is constant over frequency, or frequencyselective.

The precoder matrix is often chosen to match the characteristics of theN_(R)×N_(T) MIMO channel matrix H_(n), which may be referred to aschannel dependent precoding. This is also commonly referred to asclosed-loop precoding and essentially attempts to focus the transmitenergy into a subspace which is strong in the sense of conveying much ofthe transmitted energy to the UE. In addition, the precoder matrix mayalso be selected to orthogonalize the channel, meaning that after properlinear equalization at the UE, the inter-layer interference is reduced.

The transmission rank, and thus the number of spatially multiplexedlayers, is reflected in the number of columns of the precoder. Forefficient performance, a transmission rank may be selected to match thechannel properties.

MIMO includes single user MIMO and multi user MIMO. Transmitting all thedata layers to one UE is referred to as single user MIMO (SU-MIMO).Transmitting the data layers to multiple UEs is referred to asmulti-user MIMO (MU-MIMO).

MU-MIMO is possible when, for example, two UEs are located in differentareas of a cell such that they can be separated through differentprecoders (or beamforming) at the base transceiver station (BTS), i.e.,base station (BS). The two UEs may be served on the same time-frequencyresources (i.e., PRBs) by using different precoders or beams.

In Demodulation Reference Signal (DMRS) based transmission modes TM9 andTM10, different DMRS ports and/or the same DMRS port with differentscrambling codes can be assigned to the different UEs for MU-MIMOtransmission. In this case, MU-MIMO is transparent to the UE (i.e., a UEis not informed about the co-scheduling of another UE in the same PRBs).MU-MIMO requires more accurate downlink channel information than SU-MIMOfor the eNB to use precoding to separate the UEs (i.e., reducing crossinterference to the co-scheduled UEs).

LTE includes codebook based channel state information (CSI) estimationand feedback. In closed loop MIMO transmission schemes such as TM9 andTM10, a UE estimates and feeds back the downlink CSI to the eNB. The eNBuses the feedback CSI to transmit downlink data to the UE. The CSIconsists of a transmission rank indicator (RI), a precoding matrixindicator (PMI) and a channel quality indicator(s) (CQI).

A codebook of precoding matrices is used by the UE to find the bestmatch between the estimated downlink channel H_(n) and a precodingmatrix in the codebook based on certain criteria (e.g., UE throughput).The channel H_(n) is estimated based on a Non-Zero Power CSI referencesignal (NZP CSI-RS) transmitted in the downlink for TM9 and TM10.

The CQI/RI/PMI together provide the downlink channel state to the UE.This is also referred to as implicit CSI feedback because the estimationof H_(n) is not fed back directly. The CQI/RI/PMI can be wideband orsubband depending on the configured reporting mode.

The RI corresponds to a recommended number of streams that are to bespatially multiplexed and thus transmitted in parallel over the downlinkchannel. The PMI identifies a recommended precoding matrix codeword (ina codebook which contains precoders with the same number of rows as thenumber of CSI-RS ports) for the transmission, which relates to thespatial characteristics of the channel. The CQI represents a recommendedtransport block size (i.e., code rate) and LTE supports transmission ofone or two simultaneous (on different layers) transmissions of transportblocks (i.e., separately encoded blocks of information) to a UE in asubframe. There is thus a relation between a CQI and a signal tointerference and noise ratio (SINR) of the spatial stream(s) over whichthe transport block or blocks are transmitted.

LTE defines codebooks of up to 16 antenna ports. Both one dimension (1D)and two-dimension (2D) antenna arrays are supported. For LTE Rel-12 UEand earlier, only a codebook feedback for a 1D port layout is supported,with 2, 4 or 8 antenna ports. Thus, the codebook is designed assumingthe ports are arranged on a straight line. In LTE Rel-13, codebooks for2D port layouts were specified for the case of 8, 12, or 16 antennaports. In addition, a codebook 1D port layout for the case of 16 antennaports was also specified in LTE Rel-13.

LTE Rel-13 includes two types of CSI reporting: Class A and Class B. InClass A CSI reporting, a UE measures and reports CSI based on a newcodebook for the configured 2D antenna array with 8, 12 or 16 antennaports. The CSI consists of a RI, a PMI and a CQI or CQIs, similar to theCSI reporting in pre Rel-13.

In Class B CSI reporting, in one scenario (referred to as “K>1”), theeNB may pre-form multiple beams in one antenna dimension. There can bemultiple ports (1, 2, 4, or 8 ports) within each beam on the otherantenna dimension. Beamformed CSI-RS are transmitted along each beam. AUE first selects the best beam from a group of beams configured and thenmeasures CSI within the selected beam based on the legacy codebook for2, 4, or 8 ports. The UE then reports back the selected beam index andthe CSI corresponding to the selected beam.

In another scenario (referred to as “K=1”), the eNB may form up to 4(2D) beams on each polarization and beamformed CSI-RS is transmittedalong each beam. A UE measures CSI on the beamformed CSI-RS and feedbackCSI based on a new Class B codebook for 2, 4, 8 ports.

LTE supports two types of CSI feedbacks: period feedback and aperiodicfeedback. In periodic CSI feedback, a UE is configured to report CSIperiodically on certain preconfigured subframes. The feedbackinformation is carried on the uplink PUCCH channel.

In aperiodic CSI feedback, a UE only reports CSI when it is requested.The request is signaled on an uplink grant (i.e., either in DCI 0 or DCI4 carried on PDCCH or EPDCCH).

LTE Release-10 includes a new reference symbol sequence to estimatechannel state information referred to as non-zero power (NZP) CSI-RS.The NZP CSI-RS provides several advantages over basing the CSI feedbackon the cell-specific reference symbols (CRS) that were used for thatpurpose in previous releases.

As one example, the NZP CSI-RS is not used for demodulation of the datasignal, and thus does not require the same density (i.e., the overheadof the NZP CSI-RS is substantially less). As another example, NZP CSI-RSprovides a more flexible means to configure CSI feedback measurements(e.g., which NZP CSI-RS resource to measure on can be configured in a UEspecific manner). By measuring on a NZP CSI-RS, a UE can estimate theeffective channel that the NZP CSI-RS is traversing, including the radiopropagation channel and antenna gains.

Up to eight NZP CSI-RS ports can be configured for a LTE Rel-11 UE. TheUE can estimate the channel from up to eight transmit antenna ports inLTE Rel-11. Up to LTE Rel-12, the NZP CSI-RS utilizes an orthogonalcover code (OCC) of length two to overlay two antenna ports on twoconsecutive REs. OCC may be interchangeably referred to as code divisionmultiplexing (CDM).

Many different NZP CSI-RS patterns are available. Examples areillustrated in FIG. 5 .

FIG. 5 illustrates resource element grids with resource block pairsshowing potential positions for CSI-RS for 2, 4, and 8 antenna ports.Each resource element grid represents one PRB 16. The horizontal axisrepresents the time domain and the vertical axis represents thefrequency domain.

For 2 CSI-RS antenna ports, FIG. 5 illustrates the 20 different patternswithin a subframe (i.e., the 20 pairs of resource elements labelled 0and 1). One example pattern is illustrated with cross-hatching.

For 4 CSI-RS antenna ports, the corresponding number of patterns is 10(i.e., the 10 groups of resource elements labelled 0-3, where resourceelement pair 0 and 1 and resource element pair 2 and 3 within the samegroup are separated by 6 resource elements in the frequency domain). Oneexample pattern is illustrated with cross-hatching.

For 8 CSI-RS antenna ports, the corresponding number of patterns is 5(i.e., the 5 groups of resource elements labelled 0-7, where resourceelement pair 0 and 1 and resource element pair 2 and 3 within the samegroup are separated by 6 resource elements in the frequency domain andresource element pair 4 and 5 and resource element pair 6 and 7 withinthe same group are separated by 6 resource elements in the frequencydomain). One example pattern is illustrated with cross-hatching.

The illustrated examples are for frequency division duplex (FDD). Fortime division duplex (TDD), additional CSI-RS patterns are available.

The reference-signal sequence for CSI-RS is defined in Section 6.10.5.1of 3GPP TS 36.211 as

$\begin{matrix}{{{r_{l,n_{s}}(m)} = {{\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {2m} \right)}}} \right)} + {j\frac{1}{\sqrt{2}}\left( {1 - {2 \cdot {c\left( {{2m} + 1} \right)}}} \right)}}},} & {{Equation}\mspace{14mu} 2} \\{{m = 0},1,\ldots\mspace{14mu},{N_{RB}^{\max,{DL}} - 1}} & \;\end{matrix}$where n_(s) is the slot number within a radio frame and l is the OFDMsymbol number within the slot. The pseudo-random sequence c(i) isgenerated and initialized according to Sections 7.2 and 6.10.5.1 of 3GPPTS 36.211, respectively. Furthermore, in Equation 2, N_(RB)^(max,DL)=110 is the largest downlink bandwidth configuration supportedby 3GPP TS 36.211.

In LTE Rel-13, the NZP CSI-RS resource is extended to include 12 and 16ports. Such Rel-13 NZP CSI-RS resource is obtained by aggregating threelegacy 4 port CSI-RS resources (to form a 12 port NZP CSI-RS resource)or two legacy 8 port CSI-RS resources (to form a 16 port NZP CSI-RSresource). All aggregated NZP CSI-RS resources are located in the samesubframe. Examples of forming 12 port and 16 port NZP CSI-RS resourcesare shown in FIGS. 6A and 6B, respectively.

FIGS. 6A and 6B illustrate resource element grids with resource blockpairs showing potential positions for CSI-RS for 12 and 16 antennaports, respectively. The horizontal axis represents the time domain andthe vertical axis represents the frequency domain.

FIG. 6A illustrates an example of aggregating three 4-port resources toform a 12-port NZP CSI-RS resource. Each resource element of the same4-port resource is labeled with the same number (e.g., the fourresources labeled 1 form one 4-port resource, the four resources labeled2 form a second 4-port resource, and the four resources labeled 3 form athird 4-port resource). Together, the three aggregated 4-port resourcesform a 12 port resource.

FIG. 6B illustrates an example of aggregating two 8-port resources toform a 16-port NZP CSI-RS resource. Each resource element of the same8-port resource is labeled with the same number (e.g., the eightresources labeled 1 form one 8-port resource, and the eight resourceslabeled 2 form a second 8-port resource). Together, the two aggregated8-port resources form a 16 port resource.

In a given subframe, three 12-port resource configurations (i.e., nineout of ten 4-port resources used) and two 16-port resourceconfigurations (i.e., four out of five 8-port resources used) arepossible. The following port numbering is used for the aggregated NZPCSI-RS resources. For 16 NZP CSI-RS ports, the aggregated port numbersare 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 and 30.For 12 NZP CSI-RS ports, the aggregated port numbers are 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25 and 26.

In addition, Rel-13 NZP CSI-RS design supports two different OCClengths. Multiplexing antenna ports is possible using OCC lengths twoand four for both 12-port and 16-port NZP CSI-RS. Up to Release 13 inLTE, CSI-RS is transmitted in all PRBs in the system bandwidth with adensity of 1 RE/port/PRB.

Examples using OCC length two are illustrated in FIGS. 7 and 8 .Examples using OCC length four are illustrated in FIGS. 9 and 10 .

FIG. 7 illustrates a resource element grid with an example NZP CSI-RSdesign for 12 ports with OCC length 2. The different 4-port NZP CSI-RSresources are denoted by the letters A-J. For example, 4-port resourcesA, F, and J could be aggregated to form a 12-port NZP CSI-RS resource.The length 2 OCC is applied across two resource elements with the samesub-carrier index and adjacent OFDM symbol indices (e.g., OCC 2 isapplied to the resource elements with OFDM symbol indices 5-6 andsub-carrier index 9 in slot 0).

FIG. 8 illustrates a resource element grid with an example NZP CSI-RSdesign for 16 ports with OCC length 2. The different 8-port NZP CSI-RSresources are indicated by number (e.g., 0-4). The resource pairs thatcomprise the 8-port resource are indicated by letter (e.g., A-D). Forexample, the resource pairs A0, B0, C0 and D0 form one 8-port NZP CSI-RSresource. The resource pairs A3, B3, C3 and D3 form another 8-port NZPCSI-RS resource. 8-port NZP CSI-RS resources 0 and 3, for example, maybe aggregated to form a 16-port NZP CSI-RS resource. The length 2 OCC isapplied across two resource elements with the same sub-carrier index andadjacent OFDM symbol indices (e.g., OCC 2 is applied to the resourceelements with OFDM symbol indices 2-3 and sub-carrier index 7 in slot1).

For the OCC length 2 case (i.e., when higher layer parameter ‘cdmType’is set to cdm2 or when ‘cdmType’ is not configured by Evolved UniversalTerrestrial Access Network (EUTRAN)—see 3GPP TS 36.331 for furtherdetails), the mapping of the reference signal sequence r_(l,m) _(s)^((m)) of Equation 2 to the complex-valued modulation symbols a_(k,l)^((p)) used as reference symbols on antenna port p is defined as:

$\begin{matrix}{\mspace{79mu}{a_{k,l}^{(p^{\prime})} = {w_{l^{''}} \cdot {r_{l,n_{s}}\left( m^{\prime} \right)}}}} & {{Equation}\mspace{14mu} 3} \\{\mspace{79mu}{where}} & \; \\{k = {k^{\prime} + {12m} + \left\{ \begin{matrix}{{- 0}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,{16}} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 6}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {17,18} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 1}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {19,20} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 7}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {21,22} \right\}},{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 0}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,{16}} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 3}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {17,18} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 6}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {19,20} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}} \\{{- 9}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {21,22} \right\}},{{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}}\end{matrix} \right.}} & \; \\{l = {l^{\prime} + \left\{ \begin{matrix}l^{''} & {{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0\text{-}19},} \\\; & {{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}} \\{2l^{''}} & {{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 20\text{-}31},} \\\; & {{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}} \\l^{''} & {{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0\text{-}27},} \\\; & {{extended}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \right.}} & {{Equation}\mspace{14mu} 4} \\{\mspace{79mu}{w_{l^{\prime}} = \left\{ \begin{matrix}1 & {p^{\prime} \in \left\{ {15,17,19,21} \right\}} \\\left( {- l} \right)^{l^{''}} & {p^{\prime} \in \left\{ {16,18,20,22} \right\}}\end{matrix} \right.}} & \; \\{\mspace{79mu}{{l^{''} = 0},1}} & \; \\{\mspace{79mu}{{m = 0},1,\ldots\mspace{14mu},{N_{RB}^{DL} - 1}}} & \; \\{\mspace{79mu}{m^{\prime} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} & \;\end{matrix}$

In Equations 3 and 4, N_(RB) ^(DL) represents the downlink transmissionbandwidth; the indices k′ and l′ indicate the subcarrier index (startingfrom the bottom of each PRB) and the OFDM symbol index (starting fromthe right of each slot). The mapping of different (k′, l′) pairs todifferent CSI-RS resource configurations is given in Table 1. Thequantity p′ for the case of OCC length 2 is related to the antenna portnumber p as follows:

-   -   p=p′ for CSI-RS using up to 8 antenna ports    -   when higher-layer parameter ‘cdmType’ is set to cdm2 for CSI-RS        using more than 8 antenna ports, then

$\begin{matrix}{p = \left\{ \begin{matrix}{p^{\prime} + {\frac{N_{ports}^{CSI}}{2}i}} & {{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,\ldots\mspace{14mu},} \right.} \\\; & \left. {15 + {N_{ports}^{CSI}/2} - 1} \right\} \\{p^{\prime} + {\frac{N_{ports}^{CSI}}{2}\left( {i + N_{res}^{CSI} - 1} \right)}} & {{{for}\mspace{14mu} p^{\prime}} \in \left\{ {{15 + {N_{ports}^{CSI}/2}},\ldots\mspace{14mu},} \right.} \\\; & \left. {15 + N_{ports}^{CSI} - 1} \right\}\end{matrix} \right.} & {{Equation}\mspace{14mu} 5}\end{matrix}$

-   -   wherein i∈{0, 1, . . . , N_(res) ^(CSI)−1} is the CSI resource        number; and N_(res) ^(CSI) and N_(ports) ^(CSI) respectively        denote the number of aggregated CSI-RS resources and the number        of antenna ports per aggregated CSI-RS resource. As described        above, the allowed values of N_(res) ^(CSI) and N_(ports) ^(CSI)        for the cases of 12 and 16 port NZP CSI-RS design are given in        Table 2.

TABLE 1 Mapping from CSI reference signal configuration to (k′,1′) fornormal cyclic prefix- taken from Table 6. 10.5.2-1 of 3GPP TS 36.211.Number of CSI reference signals configured 1 or 2 4 8 Normal SpecialNormal Special Normal Special CSI-RS subframe subframe subframe subframesubframe subframe config. (k′, l′) n′_(s) (k′, l′) n′_(s) (k′, l′)n′_(s) (k′, l′) n′_(s) (k′, l′) n′_(s) (k′, l′) n′_(s)  0 (9, 5) 0 (9,5) 0 (9, 5) 0 (9, 5) 0 (9, 5) 0 (9, 5) 0  1 (11, 2) 1 (11, 5) 0 (11, 2)1 (11, 5) 0 (11, 2) 1 (11, 5) 0  2 (9, 2) 1 (9, 2) 1 (9, 2) 1 (9, 2) 1(9, 2) 1 (9, 2) 1  3 (7, 2) 1 (7, 5) 0 (7, 2) 1 (7, 5) 0 (7, 2) 1 (7, 5)0  4 (9, 5) 1 (9, 5) 1 (9, 5) 1  5 (8, 5) 0 (8, 5) 0 (8, 5) 0 (8, 5) 0 6 (10, 2) 1 (10, 5) 0 (10, 2) 1 (10, 5) 0  7 (8, 2) 1 (8, 2) 1 (8, 2) 1(8, 2) 1  8 (6, 2) 1 (6, 5) 0 (6, 2) 1 (6, 5) 0  9 (8, 5) 1 (8, 5) 1 10(3, 5) 0 (3, 5) 0 11 (2, 5) 0 (2, 5) 0 12 (5, 2) 1 (5, 5) 0 13 (4, 2) 1(4, 5) 0 14 (3, 2) 1 (3, 2) 1 15 (2, 2) 1 (2, 2) 1 16 (1, 2) 1 (1, 5) 017 (0, 2) 1 (0, 5) 0 18 (3, 5) 1 19 (2, 5) 1 20 (11, 1) 1 (11, 1) 1(11, 1) 1 21 (9, 1) 1 (9, 1) 1 (9, 1) 1 22 (7, 1) 1 (7, 1) 1 (7, 1) 1 23(10, 1) 1 (10, 1) 1 24 (8, 1) 1 (8, 1) 1 25 (6, 1) 1 (6, 1) 1 26 (5, 1)1 27 (4, 1) 1 28 (3, 1) 1 29 (2, 1) 1 30 (1, 1) 1 31 (0, 1) 1 Note:n′_(s) = n_(s) mod 2. Configurations 0-19 for normal subfrarnes areavailable for frame structure types 1, 2 and 3. Configurations 20-31 andconfigurations for special subframes are available for frame structuretype 2 only.

TABLE 2 Aggregation of CSI-RS resources—taken from Table 6.10.5-1 of3GPP TS 36.211. Total number of Number of antenna Number of CSI-RSantenna ports ports per resources resources N_(res) ^(CSI) N_(ports)^(CSI) N_(ports) ^(CSI) N_(res) ^(CSI) 12 4 3 16 8 2

FIG. 9 illustrates a resource element grid with an example NZP CSI-RSdesign for 12 ports with OCC length 4. The different 4-port NZP CSI-RSresources are denoted by the letters A-J. For example, 4-port resourcesA, F, and J could be aggregated to form a 12-port NZP CSI-RS resource. Alength 4 OCC is applied within a CDM group where a CDM group consists ofthe 4 resource elements used for mapping legacy 4-port CSI-RS. That is,the resource elements labeled with the same letter comprise one CDMgroup.

FIG. 10 illustrates a resource element grid with an example NZP CSI-RSdesign for 16 ports with OCC length 4. The different 8-port NZP CSI-RSresources are indicated by number (e.g., 0-4). The resource pairs thatcomprise the 8-port resource are indicated by letter (e.g., A-B). Forexample, the resource pairs labelled A0 and B0 form one 8-port NZPCSI-RS resource. The resource pairs labelled A3 and B3 form another8-port NZP CSI-RS resource. 8-port NZP CSI-RS resources 0 and 3, forexample, may be aggregated to form a 16-port NZP CSI-RS resource. A andB are the CDM groups within each 8-port resource. An OCC with length 4is applied within each CDM group.

For the OCC length 4 case (i.e., when higher layer parameter ‘cdmType’is set to cdm4—see 3GPP TS 36.331 for further details), the mapping ofthe reference signal sequence r_(l,n) _(s) (m) of Equation 2 to thecomplex-valued modulation symbols a_(k,l) ^((p)) used as referencesymbols on antenna port p are defined as:

$\begin{matrix}{\mspace{79mu}{a_{k,l}^{(p^{\prime})} = {{w_{p^{\prime}}(i)} \cdot {r_{l,n_{s}}\left( m^{\prime} \right)}}}} & {{Equation}\mspace{14mu} 6} \\{\mspace{79mu}{where}} & \; \\{k = {k^{\prime} + {12m} - \left\{ \begin{matrix}{k^{''}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,16,19,20} \right\}},} \\\; & {{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}},{N_{ports}^{CSI} = 8}} \\{{k^{''} + 6}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {17,18,21,22} \right\}},} \\\; & {{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}},{N_{ports}^{CSI} = 8}} \\{{6k^{''}}\ } & {{{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,16,17,18} \right\}},} \\\; & {{{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}},{N_{ports}^{CSI} = 4}}\end{matrix} \right.}} & \; \\{l = {l^{\prime} + \left\{ \begin{matrix}l^{''} & {{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 0\text{-}19},} \\\; & {{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}} \\{2l^{''}} & {{{CSI}\mspace{14mu}{reference}\mspace{14mu}{signal}\mspace{14mu}{configurations}\mspace{14mu} 20\text{-}31},} \\\; & {{normal}\mspace{14mu}{cyclic}\mspace{14mu}{prefix}}\end{matrix} \right.}} & \; \\{\mspace{79mu}{{l^{''} = 0},1}} & {{{Equation}\mspace{14mu} 7}\;} \\{\mspace{79mu}{{k^{''} = 0},1}} & \; \\{\mspace{79mu}{i = {{2k^{''}} + l^{''}}}} & \; \\{\mspace{79mu}{{m = 0},1,\ldots\mspace{14mu},{N_{RB}^{DL} - 1}}} & \; \\{\mspace{79mu}{m^{\prime} = {m + \left\lfloor \frac{N_{RB}^{\max,{DL}} - N_{RB}^{DL}}{2} \right\rfloor}}} & \;\end{matrix}$In Equations 6 and 7, N_(RB) ^(DL) represents the downlink transmissionbandwidth; N_(ports) ^(CSI) denotes the number of antenna ports peraggregated CSI-RS resource; the indices k′ and l′ indicate thesubcarrier index (starting from the bottom of each RB) and the OFDMsymbol index (starting from the right of each slot). The mapping ofdifferent (k′, l′) pairs to different CSI-RS resource configurations isgiven in Table 1. Furthermore, w_(p′)(i) in Equation 6 is given by Table3. When higher-layer parameter ‘cdmType’ is set to cdm4 for CSI-RS usingmore than 8 antenna ports, antenna port number:p=i′N _(ports) ^(CSI) +p′  Equation 8where p′∈{15, 16, . . . , 15+N_(ports) ^(CSI)−1} for CSI-RS resourcenumber i′∈{0, 1, . . . , N_(res) ^(CSI)−1}.

TABLE 3 The sequence w_(p′)(i) for CDM4—taken from Table 6.10.5.2-0 of3GPP TS 36.211 p′ N_(ports) ^(CSI) = 4 N_(ports) ^(CSI) = 8 [w_(p′)(0)w_(p′)(1) w_(p′)(2) w_(p′)(3)] 15 15, 17 [1 1 1 1] 16 16, 18 [1 −1 1 −1]17 19, 21 [1 1 −1 −1] 18 20, 22 [1 −1 −1 1]

The number of different 12-port and 16-port CSI-RS configurations in asubframe in the LTE Release 13 NZP CSI-RS designs are three and two,respectively. That is, for the 12 port case, three different CSI-RSconfigurations can be formed where each configuration is formed byaggregating three legacy 4-port CSI-RS configurations. This consumes 36CSI-RS resource elements of the 40 CSI-RS resource elements availablefor CSI-RS within a PRB. For the 16 port case, two different CSI-RSconfigurations can be formed where each configuration is formed byaggregating two legacy 8-port CSI-RS configurations. This consumes 32CSI-RS resource elements of the 40 CSI-RS resource elements availablefor CSI-RS within a PRB.

In LTE, CSI-RS may be transmitted periodically on certain subframes,referred to as CSI-RS subframes. In NR, CSI-RS may be transmitted oncertain slots (i.e., CSI-RS slots). The term CSI-RS subframes may beused interchangeably to refer to CSI-RS subframes or slots. CSI-RSsubframe/slot configuration consists of a subframe/slot periodicity anda subframe/slot offset. In LTE, the periodicity is configurable at 5,10, 20, 40 and 80 ms.

A CSI-RS configuration consists of a CSI-RS resource configuration and aCSI-RS subframe configuration. A UE can be configured with up to threeCSI-RS configurations for CSI feedback.

To improve CSI-RS channel estimation, an eNB may not transmit anysignals in certain CSI-RS REs, referred as zero-power CSI-RS or ZPCSI-RS. The CSI-RS used for CSI estimation is also referred to asnon-zero power CSI-RS or NZP CSI-RS. When the ZP CSI-RS REs in a firsttransmission (on a first cell, a first beam, and/or intended for a firstUE) coincide with NZP CSI-RS REs in a second transmission (on a secondcell, a second beam, and/or intended for a second UE), the firsttransmission does not interfere with the NZP CSI-RS in the secondtransmission. By avoiding interference in this way, the CSI-RS channelestimation for a cell, beam, and/or a UE can be improved.

When a physical channel or signal is transmitted in distinct, orthogonalresources K times, this is termed a reuse factor of K. A reuse factor ofK cells for CSI-RS means that K non-overlapping (that is, not occupyingthe same REs if they occupy the same subframes) CSI-RS resources areconfigured or reserved in each cell and one of the K resources is usedby each cell.

FIG. 11 illustrates an example of reuse factor K=3 for CSI-RS. Moreparticularly, FIG. 11 shows an example of reuse factor K=3 for CSI-RS,where 3 CSI-RS resources are configured in each cell but only one CSI-RSresource is configured for NZP CSI-RS and the other two resources areconfigured as ZP CSI-RS.

The NZP CSI-RS in different cells are non-overlapping. For example, if21 of the 40 REs available for CSI-RS in a subframe are used for NZPCSI-RS by one cell, the remaining 19 CSI-RS REs are not enough forconfiguring a 20-port NZP CSI-RS in another cell. Thus, only one cellcan transmit more than 20 NZP CSI-RS in a subframe without CSI-RScollision with other cells' CSI-RS. Therefore, to achieve a K>1 reusefactor with more than 20 ports, cells must transmit their CSI-RS indifferent subframes. As discussed below, Rel-13 UEs can generally onlybe configured to receive ZP CSI-RS in one subframe out of T_(CSI-RS)subframes.

Only CSI-RS REs for 4 antenna ports can be allocated to ZP CSI-RS. A ZPCSI-RS subframe configuration is associated with ZP CSI-RS. It can bethe same as or different from a NZP CSI-RS configuration.

The subframe configuration period T CSI-RS and the subframe offsetΔ_(CSI-RS) for the occurrence of CSI reference signals are listed inTable 6.10.5.3-1 of 3GPP TS 36.211 (shown as Table 4 below). Theparameter I_(CSI-RS) in Table 4 can be configured separately for CSIreference signals for which the UE shall assume non-zero and zerotransmission power.

TABLE 4 CSI reference signal subframe configuration (Taken from Table6.10.5.3-1 of 3GPP TS 36.211) CSI-RS periodicity CSI-RS subframe offsetCSI-RS-SubframeConfig T_(CSI-RS) Δ_(CSI-RS) I_(CSI-RS) (subframes)(subframes) 0-4 5 I_(CSI-RS)  5-14 10 I_(CSI-RS)-5 15-34 20I_(CSI-RS)-15 35-74 40 I_(CSI-RS)-35 75-154 80 I_(CSI-RS)-75

In general, a full dimension MIMO (FD-MIMO) UE (one configured for ClassA or Class B CSI reporting) can only be configured with one ZP CSI-RSconfiguration. UEs that support discovery signal reception or receptionusing two subframe sets can support more than one ZP CSI-RSconfiguration. FD-MIMO UEs are not required to support these features.

Because FD-MIMO UEs only support up to 16 CSI-RS ports in Rel-13, areuse factor of up to 2 can be supported while using ZP CSI-RS toprotect NZP CSI-RS, because two sets of 16 ports can fit in onesubframe, as described above. However, Rel-14 may include up to 32ports, and because only one ZP CSI-RS can be configured for certainFD-MIMO UEs in Rel-13, Rel-13 mechanisms that protect more than one NZPCSI-RS with a ZP CSI-RS in different subframes are not available. Thus,reuse patterns >2 are not possible for these UEs with 32 ports.

TM10 includes a concept referred to as a CSI process (see 3GPP TS36.213). A CSI process is associated with a NZP CSI-RS resource and aCSI interference measurement (CSI-IM) resource. A CSI-IM resource isdefined by a ZP CSI-RS resource and a ZP CSI-RS subframe configuration.A UE can be configured with up to 3 CSI-RS processes. Multiple CSIprocesses are used to support Coordinated Multi-Point (COMP)transmission in which a UE measures and feeds back CSI associated witheach transmission point (TP) to an eNB. Based on the received CSIs, theeNB may decide to transmit data to the UE from one of the TPs.

In Rel-13, a CSI-IM is always associated with an NZP CSI-RS resource ina one-to-one fashion, such that the number of CSI-IMs is equal to thenumber of NZP CSI-RS resources. Therefore, while a CSI-IM is constructedfrom a ZP CSI-RS resource, it is not suitable for preventinginterference to other NZP CSI-RS, and so is not useful to increaseCSI-RS reuse factors.

Measurement restriction is included in LTE Release 13 for TM9 and TM10.CSI measurement may be restricted to a CSI-RS resource or a CSI-IMresource in one subframe.

For a UE in TM9 or TM10 and for a CSI process, if the UE is configuredwith parameter CSI-Reporting-Type by higher layers, andCSI-Reporting-Type is set to ‘CLASS B’, and parameterchannelMeasRestriction is configured by higher layers, the UE shallderive the channel measurements for computing the CQI value reported inuplink subframe n and corresponding to the CSI process, based on onlythe most recent, no later than the CSI reference resource, non-zeropower CSI-RS within a configured CSI-RS resource associated with the CSIprocess.

For a UE in TM10 and for a CSI process, when parametersCSI-Reporting-Type and interferenceMeasRestriction is configured byhigher layers, the UE shall derive the interference measurements forcomputing the CQI value reported in uplink subframe n and correspondingto the CSI process, based on only the most recent, no later than the CSIreference resource, configured CSI-IM resource associated with the CSIprocess.

Channel measurement restriction to one CSI-RS subframe is needed inClass B in which the precoding for CSI-RS may be different in differentCSI-RS subframes.

In LTE Rel-14, up to 32 antenna ports may be supported in the downlink.However, a maximum of 40 CSI-RS REs are available per PRB in a CSI-RSsubframe. Thus, only one 32 port CSI-RS configuration can be supportedper CSI-RS subframe. How to reduce CSI-RS overhead and mechanisms on howto increase the number to allow higher number of CSI-RS configurationswith 32 ports is discussed in 3GPP TSG-RAN R1-163079, “CSI-RS Design forClass A eFD-MIMO.” Measurement restriction (MR) in frequency domaindescribed in R1-163079 is one technique that may achieve these goals.

Although the general concept of MR in frequency domain is described inR1-163079, some details of this technique are still missing. Forexample, one problem is that when the measurement restriction isconfigured, how the UE interprets resource element to port mapping isnot clearly defined.

As shown in Table 4 above, ZP CSI-RS for a serving cell can only beconfigured with a single CSI-RS-SubframeConfig parameter if a UE onlysupports one ZP CSI-RS configuration. This means that the ZP CSI-RS canonly happen in one given subframe configuration. However, withincreasing number of CSI-RS ports being available in LTE Release 14,achieving reuse factors higher than 1 for CSI-RS within a singlesubframe is not possible for large numbers of CSI-RS ports, such as 32CSI-RS ports.

SUMMARY

The embodiments described herein include resource element (RE) to portmapping for measurement restriction (MR) in frequency domain (FD).Particular embodiments include a port mapping scheme for the case wherean eNB semi-statically configures a user equipment (UE) to measure allports on a subset of physical resource blocks (PRBs). Some embodimentsinclude various alternatives for how the MR sets may be signaled.

Particular embodiments include a port mapping scheme for the case wherea UE is semi-statically configured to measure channels on a subset ofCSI-RS ports on one set of PRBs and another subset of antenna ports on adifferent set of PRBs. Various alternatives for how the MR sets andCSI-RS resource sets which contain subset of ports are described.Particular embodiments describe how RE to port mapping is performedusing the RRC configured MR_set and/or CSI-RS Resource set parameters ina configurable manner.

Some embodiments include multiple ZP CSI-RS subframe configurations. Forexample, particular embodiments include configuring multiple ZP CSI-RSsubframe configurations to enable higher reuse factors.

According to some embodiments, a method for use in a network node oftransmitting channel state information reference signals (CSI-RS)comprises transmitting, to a wireless device, an indication of a subsetof physical resource blocks (PRBs) that the wireless device should useto measure CSI-RS. Each CSI-RS is associated with an antenna port. Thesubset of PRBs comprises a subset of the system bandwidth. The methodfurther comprises transmitting CSI-RS on the indicated subset of PRBs.

The method may further comprise, prior to transmitting the indication,obtaining, by the network node, the indication of the subset of physicalresource blocks (PRBs) that the wireless device should use to measureCSI-RS.

In particular embodiments, the network node transmits CSI-RS on a totalnumber of antenna ports (e.g., greater than 16), and each PRB of thesubset of PRBs includes a CSI-RS mapping for the total number of antennaports.

In particular embodiments, the subset of PRBs that the wireless deviceshould use to measure CSI-RS comprises even numbered PRBs or oddnumbered PRBs. The indication of the subset of PRBs that the wirelessdevice should use to measure CSI-RS may comprise a density value and acomb offset.

For example, the density value may comprise a density of ½. A first comboffset indicates that the wireless device should use the PRBs in, orrather identified by, a set m1 to measure CSI-RS, wherein the set m1comprises {0, 2, . . . , N_(RB) ^(DL)−2}. A second comb offset indicatesthat the wireless device should use the PRBs in, or rather identifiedby, a set m2 to measure CSI-RS, wherein the set m2 comprises {1, 3, . .. , N_(RB) ^(DL)−1}.

As another example, the density value may comprise a density of ⅓. Afirst comb offset indicates that the wireless device should use the PRBsin, or rather identified by, a set m1 to measure CSI-RS, wherein the setm1 comprises {0, 3, . . . , N_(RB) ^(DL)−3}. A second comb offsetindicates that the wireless device should use the PRBs in, or ratheridentified by, a set m2 to measure CSI-RS, wherein the set m2 comprises{1, 4, . . . , N_(RB) ^(DL)−2}. A third comb offset indicates that thewireless device should use the PRBs in, or rather identified by, a setm3 to measure CSI-RS, wherein the set m3 comprises {2, 5, . . . , N_(RB)^(DL)−1}.

The indication of the subset of PRBs that the wireless device should useto measure CSI-RS ports may comprise an index value k. The index value krefers to one of a plurality of indications stored at the wirelessdevice.

In particular embodiments, the indication of the subset of PRBs that thewireless device should use to measure CSI-RS ports further comprises anumber of successive CSI-RS subframes/slots in which the wireless deviceshould measure CSI-RS.

The method may further comprise receiving, from the wireless device, achannel state information (CSI) determined based on measurements of oneor more of the transmitted CSI-RS.

According to some embodiments, a method for use in a wireless device ofreceiving CSI-RS comprises receiving an indication of a subset of PRBsthat the wireless device should use to measure CSI-RS. Each of theCSI-RS is associated with an antenna port. The subset of PRBs comprisesa subset of the system bandwidth. The method further comprises receivingCSI-RS on the indicated subset of PRBs. The method may further comprisedetermining a CSI based on the received CSI-RS and transmitting the CSIto a network node.

In particular embodiments, a network node transmits CSI-RS on a totalnumber of antenna ports (e.g., greater than 16), and each PRB of thesubset of PRBs includes a CSI-RS mapping for the total number of antennaports.

In particular embodiments, the subset of PRBs that the wireless deviceshould use to measure CSI-RS comprises even numbered PRBs or oddnumbered PRBs. The indication of the subset of PRBs that the wirelessdevice should use to measure CSI-RS ports may comprise a density valueand a comb offset.

For example, the density value may comprise a density of ½. A first comboffset indicates that the wireless device should use the PRBs in, orrather identified by, a set m1 to measure CSI-RS, wherein the set m1comprises {0, 2, . . . , N_(RB) ^(DL)−2}. A second comb offset indicatesthat the wireless device should use the PRBs in, or rather identifiedby, a set m2 to measure CSI-RS, wherein the set m2 comprises {1, 3, . .. , N_(RB) ^(DL)−1}.

As another example, the density value may comprise a density of ⅓. Afirst comb offset indicates that the wireless device should use the PRBsin, or rather identified by, a set m1 to measure CSI-RS, wherein the setm1 comprises {0, 3, . . . , N_(RB) ^(DL)−3}. A second comb offsetindicates that the wireless device should use the PRBs in, or ratheridentified by, a set m2 to measure CSI-RS, wherein the set m2 comprises{1, 4, . . . , N_(RB) ^(DL)−2}. A third comb offset indicates that thewireless device should use the PRBs in, or rather identified by, a setm3 to measure CSI-RS, wherein the set m3 comprises {2, 5, . . . , N_(RB)^(DL)−1}.

The indication of the subset of PRBs that the wireless device should useto measure CSI-RS ports may comprise an index value k. The index value krefers to one of a plurality of indications stored at the wirelessdevice.

In particular embodiments, the indication of the subset of PRBs that thewireless device should use to measure CSI-RS ports further comprises anumber of successive CSI-RS subframes/slots in which the wireless deviceshould measure CSI-RS.

In particular embodiments, the method further comprises determining aCSI based on the received CSI-RS over the number of successive CSI-RSsubframes/slots.

According to some embodiments, a network node operable to transmitCSI-RS comprises processing circuitry. The processing circuitry isoperable to transmit, to a wireless device, an indication of a subset ofPRBs that the wireless device should use to measure CSI-RS. Each CSI-RSis associated with an antenna port. The subset of PRBs comprises asubset of the system bandwidth. The processing circuitry is furtheroperable to transmit CSI-RS on the indicated subset of PRBs.

The processing circuitry may further be operable to, prior totransmitting the indication, obtain the indication of the subset ofphysical resource blocks (PRBs) that the wireless device should use tomeasure CSI-RS.

According to some embodiments, a wireless device operable to receiveCSI-RS comprises processing circuitry. The processing circuitry operableto receive an indication of a subset of PRBs that the wireless deviceshould use to measure CSI-RS. Each CSI-RS is associated with an antennaport. The subset of PRBs comprises a subset of the system bandwidth. Theprocessing circuitry is further operable to receive CSI-RS on theindicated subset of PRBs.

According to some embodiments, a network node operable to transmitCSI-RS comprises a transmitting module. The network node may furthercomprise an obtaining module. The obtaining module is operable to obtainan indication of a subset of PRBs that a wireless device should use tomeasure CSI-RS. The transmitting module is operable to: transmit, to thewireless device, the indication of the subset of PRBs that the wirelessdevice should use to measure CSI-RS; and transmit CSI-RS on theindicated subset of PRBs.

According to some embodiments, a wireless device operable to receiveCSI-RS comprises a receiving module. The receiving module is operableto: receive an indication of a subset of PRBs that the wireless deviceshould use to measure CSI-RS; and receive CSI-RS on the indicated subsetof PRBs.

Also disclosed is a computer program product. The computer programproduct comprises instructions stored on non-transient computer-readablemedia which, when executed by a processor, perform the acts oftransmitting, to the wireless device, the indication of the subset ofPRBs that the wireless device should use to measure CSI-RS; andtransmitting CSI-RS on the indicated subset of PRBs. The computerprogram product may further comprise instructions stored onnon-transient computer-readable media which, when executed by theprocessor, perform the acts of obtaining the indication of the subset ofPRBs that the wireless device should use to measure CSI-RS.

Another computer program product comprises instructions stored onnon-transient computer-readable media which, when executed by aprocessor, perform the acts of receiving an indication of a subset ofPRBs that the wireless device should use to measure CSI-RS; andreceiving CSI-RS on the indicated subset of PRBs.

Particular embodiments may exhibit some of the following technicaladvantages. As one example, certain embodiments may enable themeasurement restriction in frequency domain technique by using efficientand flexible RE to port mapping schemes. As another example, certainembodiments may enable different CSI-RS ports to have different CSI-RSdensity in the frequency domain. As yet another example, certainembodiments may enable higher reuse factors for CSI-RS transmissionswith a higher number of ports (e.g., 32 ports). Other technicaladvantages will be readily apparent to one skilled in the art from thefollowing figures, description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the embodiments and their featuresand advantages, reference is now made to the following description,taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates an example downlink radio subframe;

FIG. 2 illustrates an example radio frame;

FIG. 3 illustrates an example downlink subframe;

FIG. 4 is a block diagram illustrating the logical structure of precodedspatial multiplexing mode in LTE;

FIG. 5 illustrates resource element grids with resource block pairsshowing potential positions for CSI-RS for 2, 4, and 8 antenna ports;

FIGS. 6A and 6B illustrate resource element grids with resource blockpairs showing potential positions for CSI-RS for 12 and 16 antennaports, respectively;

FIG. 7 illustrates a resource element grid with an example NZP CSI-RSdesign for 12 ports with OCC length 2;

FIG. 8 illustrates a resource element grid with an example NZP CSI-RSdesign for 16 ports with OCC length 2;

FIG. 9 illustrates a resource element grid with an example NZP CSI-RSdesign for 12 ports with OCC length 4;

FIG. 10 illustrates a resource element grid with an example NZP CSI-RSdesign for 16 ports with OCC length 4;

FIG. 11 illustrates an example of reuse factor K=3 for CSI-RS;

FIG. 12 is a block diagram illustrating an example wireless network,according to some embodiments;

FIG. 13 illustrates an example of CSI-RS transmission where all portsare transmitted on one restricted set of PRBs, in accordance withcertain embodiments;

FIG. 14 illustrates an example of CSI-RS transmission where one set ofports are transmitted on one restricted set of PRBs and another set ofports are transmitted on another restricted set of PRBs, in accordancewith certain embodiments;

FIG. 15 illustrates an example of CSI-RS transmission where one set ofports are transmitted on all PRBs and another set of ports aretransmitted on a restricted set of PRBs, in accordance with certainembodiments;

FIG. 16 illustrates an example of configuring multiple ZP CSI-RSsubframes, in accordance with certain embodiments;

FIG. 17 is a flow diagram illustrating an example method in a networknode of transmitting channel state information reference signals(CSI-RS), according to some embodiments;

FIG. 18 is a flow diagram illustrating an example method in a wirelessdevice of receiving channel state information reference signals(CSI-RS), according to some embodiments;

FIG. 19A is a block diagram illustrating an example embodiment of awireless device;

FIG. 19B is a block diagram illustrating example components of awireless device;

FIG. 20A is a block diagram illustrating an example embodiment of anetwork node;

FIG. 20B is a block diagram illustrating example components of a networknode; and

FIGS. 21A and B illustrate examples of overhead for a TDM scheme.

DETAILED DESCRIPTION

Third Generation Partnership Project (3GPP) Long Term Evolution (LTE)uses Non-Zero Power Channel State Information Reference Symbols (NZPCSI-RS) as a flexible means to configure channel state information (CSI)feedback measurements. By measuring on a NZP CSI-RS, a user equipment(UE) can estimate the effective channel the NZP CSI-RS is traversing,including the radio propagation channel and antenna gains.

In LTE Rel-14, up to 32 antenna ports may be supported in the downlink.However, a maximum of 40 CSI-RS resource elements (REs) are availableper physical resource block (PRB) in a CSI-RS subframe. Thus, only one32 port CSI-RS configuration can be supported per CSI-RS subframe.Particular embodiments obviate the problems described above and mayreduce CSI-RS overhead and facilitate a higher number of CSI-RSconfigurations with 32 ports.

In certain embodiments, methods are described for establishing aresource element to port mapping for measurement restriction infrequency domain. According to one example embodiment, a port mappingscheme is described for the case where an eNB semi-statically configuresthe UE to measure all ports on a subset of PRBs. Various alternativesfor how the measurement restriction sets could be signaled aredescribed. According to another example embodiment, a port mappingscheme is proposed for the case where a UE is semi-statically configuredto measure channels on a subset of CSI-RS ports on one set of PRBs andanother subset of antenna ports on a different set of PRBs. Variousalternatives are described for how particular antenna ports are assignedto the measurement restriction sets and/or CSI-RS resource sets. Incertain embodiments, solutions for how RE to port mapping may beperformed using the RRC configured MR_set and/or CSI-RS resource setparameters in a configurable manner are described.

In certain embodiments, the network indicates to a UE if the UE canassume that a first PRB in a slot of a subframe contains a CSI-RS porttransmission and that a second PRB in the slot of the subframe does notcontain a CSI-RS port transmission, wherein the CSI-RS port isidentified by a non-negative integer. In some cases, the first PRB maybe identified with an index m₁=Nk, and the second PRB may be identifiedwith m₂=Nk+n where n∈{1, 2, . . . , N−1} and k is a non-negativeinteger. In some cases, the first PRB may be identified with an indexm_(′1) that is in a set M and the second PRB may be identified with anindex m_(′2) that is not in the set M, and wherein the network signalsthe set M to the UE.

PRBs included in various alternative subsets of PRBs as discussed hereinmay thus be identified by their respective PRB indices, and a set ofsuch PRB indices may be used to define a subset of PRBs. The subset ofPRBs is a smaller set as compared to a set of PRBs spanning the systembandwidth of the wireless network, e.g. an LTE system bandwidth or an NRsystem bandwidth. The subset of PRBs may include PRBs that are alsoincluded in the set of PRBs spanning the system bandwidth. As usedherein, PRBs may sometimes be referred to as included in a set ofindices (such as a set m1, m2 or m3 of indices), when strictly speaking,the PRB index identifying the PRB is included in the set of indices.This wording is merely used for simplicity and is not intended to belimiting.

In certain embodiments, methods are described for establishing multipleZP CSI-RS subframe configurations. According to one example embodiment,a solution of configuring multiple ZP CSI-RS subframe configurations toenable higher reuse factors is described. As described in more detailbelow, in some cases a method of avoiding interference to NZP CSI-RS notintended for a UE comprises a network configuring a UE with one NZPCSI-RS resource, and a first and a second zero power (ZP) CSI-RSresource occurring in a first and a second subframe, wherein at leastone of the first and second ZP CSI-RS resources has a periodicity of Psubframes, and the first and second subframes are distinct within theperiod P. In some cases, a method of avoiding interference to NZP CSI-RSnot intended for a UE, comprises a network configuring a UE to receiveone NZP CSI-RS and a first and a second zero power CSI-RS. The firstzero power CSI-RS occurs in a first subframe and the second zero powerCSI-RS occurs in a second subframe.

The various embodiments described herein may have one or more technicaladvantages. As one example, certain embodiments may enable themeasurement restriction in frequency domain technique by proposingefficient/flexible RE to port mapping schemes. As another example,certain embodiments may enable higher reuse factors for CSI-RStransmissions with a higher number of ports (e.g., 32 ports).

The following description sets forth numerous specific details. It isunderstood, however, that embodiments may be practiced without thesespecific details. In other instances, well-known circuits, structuresand techniques have not been shown in detail in order not to obscure theunderstanding of this description. Those of ordinary skill in the art,with the included descriptions, will be able to implement appropriatefunctionality without undue experimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to implement such feature, structure, orcharacteristic in connection with other embodiments, whether or notexplicitly described.

Although terminology from 3GPP LTE is used herein to describe particularembodiments, the embodiments are not limited to only the aforementionedsystem. Other wireless systems, including New radio (NR), Wideband CodeDivision Multiple Access (WCDMA), Worldwide Interoperability forMicrowave Access (WiMax), Ultra-Mobile Broadband (UMB) and Global Systemfor Mobile Communication (GSM), etc. may also benefit from theembodiments described herein.

Terminology such as eNodeB and UE should be considered non-limiting anddo not imply a particular hierarchical relation between the two. In NRthe corresponding node to the eNodeB is referred to as a gNodeB. Ingeneral, “eNodeB” may be considered as a first device and “UE” as asecond device. The two devices communicate with each other over a radiochannel. While particular embodiments describe wireless transmissions inthe downlink, other embodiments are equally applicable in the uplink.

Particular embodiments are described with reference to FIGS. 12-20B ofthe drawings, like numerals being used for like and corresponding partsof the various drawings. LTE is used throughout this disclosure as anexample cellular system, but the ideas presented herein may apply toother wireless communication systems as well.

FIG. 12 is a block diagram illustrating an example wireless network,according to a particular embodiment. Wireless network 100 includes oneor more wireless devices 110 (such as mobile phones, smart phones,laptop computers, tablet computers, MTC devices, or any other devicesthat can provide wireless communication) and a plurality of networknodes 120 (such as base stations or eNodeBs). Wireless device 110 mayalso be referred to as a UE. Network node 120 serves coverage area 115(also referred to as cell 115).

In general, wireless devices 110 that are within coverage of networknode 120 (e.g., within cell 115 served by network node 120) communicatewith network node 120 by transmitting and receiving wireless signals130. For example, wireless devices 110 and network node 120 maycommunicate wireless signals 130 containing voice traffic, data traffic,and/or control signals. A network node 120 communicating voice traffic,data traffic, and/or control signals to wireless device 110 may bereferred to as a serving network node 120 for the wireless device 110.Communication between wireless device 110 and network node 120 may bereferred to as cellular communication. Wireless signals 130 may includeboth downlink transmissions (from network node 120 to wireless devices110) and uplink transmissions (from wireless devices 110 to network node120).

Network node 120 and wireless device 110 may communicate wirelesssignals 130 according to a radio frame and subframe structure similar tothat described with respect to FIGS. 1-3 . Other embodiments may includeany suitable radio frame structure. For example, in NR the duration ofthe time symbols (such as OFDM symbols) may vary with the usednumerology, and a subframe may thus not always contain the same numberof symbols. Instead, the concept of “slots” may be used, a slot usuallyoccupying 14 symbols, or occasionally 7 symbols, thus corresponding toan LTE subframe.

Each network node 120 may have a single transmitter 140 or multipletransmitters 140 for transmitting signals 130 to wireless devices 110.In some embodiments, network node 120 may comprise a multiple-inputmultiple-output (MIMO) system. Similarly, each wireless device 110 mayhave a single receiver or multiple receivers for receiving signals 130from network nodes 120 or other wireless devices 110. The multipletransmitters of network node 120 may be associated with logical antennaports.

Wireless signals 130 may include reference signals, such as CSI-RSreference signals 135. In particular embodiments, wireless signals 130may include more than sixteen CSI-RS 135 in a subframe. Each CSI-RS 135may be associated with an antenna port.

In particular embodiments, a network node, such as network node 120,transmits a number of CSI-RS 135 to one or more wireless devices, suchas wireless device 110. In particular embodiments, the number of CSI-RSports, i.e. ports upon which CSI-RS 135 are transmitted, is greater than16. For example, the number of CSI-RS ports may be 32.

In particular embodiments, network node 120 may obtain an indication ofa subset of PRBs that wireless device 110 should use to measure CSI-RS.Network node 120 may transmit, to wireless device 110, the indication ofthe subset of PRBs that the wireless device 110 should use to measureCSI-RS.

In particular embodiments, network node 120 may transmit the indicationof PRBs to wireless device 110 as an indication of PRB indices (e.g.,odd or even numbered PRBs), as density value and comb offset (e.g.,density ½ with two comb offsets, density ⅓ with three comb offsets,etc.), or as an index value representing an indication or pattern ofPRBs known to the wireless device 110 (e.g., index k, where k identifiesa particular PRB pattern known to wireless device 110).

In particular embodiments, network node 120 may transmit to wirelessdevice 110 an indication of a number of successive subframes that thewireless device should use measure CSI-RS. In particular embodiments,network node 120 may receive, from wireless device 110, a channel stateinformation (CSI) based on one or more of the transmitted CSI-RS 135.

According to some embodiments, a wireless device, such as wirelessdevice 110, receives an indication of a subset of PRBs that wirelessdevice 110 should use to measure CSI-RS. Wireless device 110 receivesCSI-RS on the indicated subset of PRBs. Wireless device 110 maydetermine a channel state information (CSI) based on the received CSI-RS(i.e., measure the received CSI-RS to estimate an effective channel) andtransmit the CSI to the network node 120.

In wireless network 100, each network node 120 may use any suitableradio access technology, such as long term evolution (LTE),LTE-Advanced, Universal Mobile Telecommunications System (UMTS), HighSpeed Packet Access (HSPA), GSM, cdma2000, NR, WiMax, Wireless Fidelity(WiFi), and/or other suitable radio access technology. Wireless network100 may include any suitable combination of one or more radio accesstechnologies. For purposes of example, various embodiments may bedescribed within the context of certain radio access technologies.However, the scope of the disclosure is not limited to the examples andother embodiments could use different radio access technologies.

As described above, embodiments of a wireless network may include one ormore wireless devices and one or more different types of network nodesor radio network nodes capable of communicating with the wirelessdevices. The network may also include any additional elements suitableto support communication between wireless devices or between a wirelessdevice and another communication device (such as a landline telephone).A wireless device may include any suitable combination of hardwareand/or software. For example, in particular embodiments, a wirelessdevice, such as wireless device 110, may include the componentsdescribed with respect to FIG. 19A below. Similarly, a network node mayinclude any suitable combination of hardware and/or software. Forexample, in particular embodiments, a network node, such as network node120, may include the components described with respect to FIG. 20Abelow.

According to a first group of example embodiments, an eNBsemi-statically configures a UE to measure all ports on a subset ofPRBs. NZP CSI-RS corresponding to all ports are transmitted only on theconfigured PRBs. The eNB Radio Resource Control (RRC) configures the UEwith a frequency domain measurement restriction parameter MR_Set whichcontains all the PRBs that the UE should measure CSI-RS ports on. Anexample of this embodiment is shown in FIG. 13 .

FIG. 13 illustrates an example of CSI-RS transmission where all portsare transmitted on one restricted set of PRBs, in accordance withcertain embodiments. In the illustrated example, the system bandwidth,such as the system bandwidth of network 100 described above, comprisesN_(RB) ^(DL)−1 number of PRBs 16. PRBs 16 a represent PRBs with no NZPCSI-RS transmission. PRBs 16 b represent PRBs for CSI measurement withNZP CSI-RS transmission.

A parameter MR_Set contains the PRBs 0, 2, 4, . . . , N_(RB) ^(DL)−2(i.e., MR_Set={0, 2, 4, . . . , N_(RB) ^(DL)−2}). The PRB index m in theRE to port mapping formulas in Equation 4 and Equation 7 above ismodified as follows:m∈MR_Set  Equation 9

Alternatively stated, Equation 9 indicates to the UE, such as wirelessdevice 110, that the UE can assume that NZP CSI-RSs corresponding to allNZP CSI-RS ports are transmitted in the PRBs indicated by the setMR_Set, but not necessarily in other PRBs. Therefore, the UE shouldmeasure the channel corresponding to the ports in the indicated PRBs.Because the NZP CSI-RS can be transmitted to a UE in a configurablesubset of PRBs, the overhead associated with CSI-RS may advantageouslybe reduced in a configurable way for different deployment scenarios andload conditions.

In some embodiments, the RRC parameter MR_Set may be signaled as abitmap of length N_(RB) ^(DL) wherein the m^(th) bit indicates whetheror not NZP CSI-RS is transmitted on the m^(th) PRB. In certainembodiments, the UE is configured to use the value of MR_Set wheneverthe NZP CSI-RS is transmitted.

In some embodiments, the set of PRB indices are integers that identifywhich PRBs contain at least one NZP CSI-RS. In a particular embodiment,the integers each comprise the physical resource block number, n_(PRB),as defined in section 6.2.3 of 3GPP TS 36.211.

In the example illustrated in FIG. 13 , PRBs for CSI measurement includethe even numbered PRBs, and PRBs with no NZP CSI-RS transmission includethe odd numbered PRBs. In some embodiments, the RRC parameter MR_Set maybe signaled as a value indicating odd or even. The same pattern may beindicated by a combination of density and comb offset. For example, theillustrated example includes a density of ½ (i.e., half the PRBs includeCSI-RS and half do not). A first comb offset may indicate that the evennumbered PRBs include CS-RS and the odd numbered PRBs do not. A secondcomb offset may indicate that the odd numbered PRBs include CS-RS andthe even numbered PRBs do not. The illustrated example is but oneexample. Other embodiments may use other densities and comb offsets(e.g., density ⅓ with 3 comb offsets, etc.).

In some embodiments, a UE may build up a channel estimate over theentire band, or a portion thereof, by looking at successive CSI-RSsubframes (i.e., subframes that contain CSI-RS) where a different MR_Setis applied over the successive subframes. This may be particularlyuseful for UEs with low mobility. The eNB may signal over how maysuccessive subframes the UE should measure CSI-RS.

In some embodiments, a fixed number of measurement restriction (MR)patterns in the frequency domain may be predefined. For example, a firstmeasurement restriction pattern in frequency domain may contain NZPCSI-RS in every 2^(nd) PRB, a second measurement restriction pattern infrequency domain may contain NZP CSI-RS in every 4^(th) PRB, and a thirdmeasurement restriction pattern in frequency domain may contain NZPCSI-RS in every PRB. This can be generalized to K different predefinedmeasurement restriction patterns in the frequency domain. The eNB maysemi-statically configure the UE to use one of the K predefinedmeasurement restriction patterns in frequency domain. For instance, ifthe k^(th) measurement restriction pattern in the frequency domain is tobe configured for a particular UE, the eNB could semi-statically signalan integer parameter with value k to the UE.

According to a second group of example embodiments, a UE issemi-statically configured to measure channels on a subset of CSI-RSports on one set of PRBs and another subset of antenna ports on adifferent set of PRBs. The eNB RRC configures the UE with a first set offrequency domain measurement restriction parameters MR_Set1 that appliesto a first set of ports in the CSI-RS resource set CSI-RS_Resource_Set1.Similarly, the eNB RRC configures the UE with a second set of frequencydomain measurement restriction parameter MR_Set2 that applies to asecond set of ports in the CSI-RS resource set CSI-RS_Resource_Set2.

The resources indicated in CSI-RS_Resource_Set1 and CSI-RS_Resource_Set2are chosen from among the resources in the parameter‘nzp-resourceConfigList’ indicated by higher layers (see 3GPP TS36.331). The RRC parameters MR_Set1 and MR_Set2 each contain a list ofPRBs in which NZP CSI-RS corresponding to the respective sets aretransmitted. An example of this embodiment with 32 CSI-RS ports is shownin FIG. 14 .

FIG. 14 illustrates an example of CSI-RS transmission where one set ofports are transmitted on one restricted set of PRBs and another set ofports are transmitted on another restricted set of PRBs, in accordancewith certain embodiments. The resource element grid illustrated in FIG.14 includes a portion of a subframe 10 with two PRBs 16.

CSI-RS ports 15-30 are transmitted in even PRBs 16, and CSI-RS ports31-46 are transmitted in odd PRBs 16. The MR_Set1 parametercorresponding to ports 15-30 contain PRBs 0, 2, 4, 6, . . . (i.e.,MR_Set1={0, 2, 4, 6, . . . }); the MR_Set2 parameter corresponding toports 31-46 contain PRBs 1, 3, 5, 7, . . . (i.e., MR_Set2={1, 3, 5, 7, .. . }). Let a quantity i″ related to the legacy CSI resource number i′in Equation 5 and Equation 8 be defined as follows:i″=2q′+i′  Equation 10

In Equation 10, q′∈{0, 1} where q′=0 corresponds to the firstCSI-RS_Resource_Set1 and q′=1 corresponds to the secondCSI-RS_Resource_Set2. In the example of FIG. 14 , the CSI-RS REs in OFDMsymbols 5-6 in slot 0 belong to an 8-port legacy resource (correspondingto legacy resource number i′=0) and the CSI-RS REs in OFDM symbols 5-6in slot 1 belong to another 8-port legacy resource (corresponding tolegacy resource number i′=1).

The quantity i″ can be used as a new CSI resource number for theresources in CSI-RS_Resource_Set1 and CSI-RS_Resource_Set2. ForCSI-RS_Resource_Set1 (where q′=0), the new CSI reference numbers arei″∈{0, 1}; and for CSI-RS_Resource_Set2 (where q′=1), the new CSIreference numbers are i″∈{2, 3} Thus, using Equation 10, the two legacy8-port resources with legacy CSI resource numbers i′∈{0, 1} in theexample of FIG. 14 are split into four new 8-port resources with new CSIreference numbers i″∈{0, 1, 2, 3}.

When higher-layer parameter ‘cdmType’ is set to cdm2 for CSI-RS usingmore than 8 antenna ports, the port numbering may be given using the newCSI reference numbers as:

$\begin{matrix}{p = \left\{ \begin{matrix}{p^{\prime} + {\frac{N_{ports}^{CSI}}{2}i^{''}}} & {{{for}\mspace{14mu} p^{\prime}} \in \left\{ {15,\ldots\mspace{14mu},} \right.} \\\; & \left. {15 + {N_{ports}^{CSI}/2} - 1} \right\} \\{p^{\prime} + {\frac{N_{ports}^{CSI}}{2}\left( {i^{''} + N_{res}^{CSI} - 1} \right)}} & {{{for}\mspace{14mu} p^{\prime}} \in \left\{ {{15 + {N_{ports}^{CSI}/2}},\ldots\mspace{14mu},} \right.} \\\; & \left. {15 + N_{ports}^{CSI} - 1} \right\}\end{matrix} \right.} & {{Equation}\mspace{14mu} 11}\end{matrix}$When higher-layer parameter ‘cdmType’ is set to cdm4 for CSI-RS usingmore than 8 antenna ports, the antenna port number may be given usingthe new CSI reference numbers as:p=i′N _(ports) ^(CSI) +p′  Equation 12For example, in the example of FIG. 14 , the new resource with CSI-RSports 15-22 corresponds to i″=0; the new resource with CSI-RS ports23-30 corresponds to i″=1; the new resource with CSI-RS ports 31-3corresponds to i″=2; and the new resource with CSI-RS ports 39-46corresponds to i″=3. As described earlier, MR_Set1 applies toCSI-RS_Resource_Set1 (corresponding to i″∈{0, 1}), and MR_Set2 appliesto CSI-RS_Resource_Set2 (corresponding to i″∈{2, 3}).

To define the RE to port mapping formula for this embodiment, the PRBindex m in

Equation 4 and Equation 7 is modified as follows:

$\begin{matrix}{m \in \left\{ \begin{matrix}{{{{MR}{\_ Set}}\; 1},} & {{{if}\mspace{14mu} i^{''}} \in {{CSI} - {{RS\_ Resource}{\_ Set}\; 1}}} \\{{{MR\_ Set}\; 2},} & {{{else}\mspace{14mu}{if}\mspace{14mu} i^{''}} \in {{CSI} - {{RS\_ Resource}{\_ Set}\; 2}}}\end{matrix} \right.} & {{Equation}\mspace{14mu} 13}\end{matrix}$where the new CSI-RS resource number i″ is defined as in Equation 10above.

In some embodiments, the PRBs in the first frequency domain measurementset MR_Set1 may partially overlap with the PRBs in the second frequencydomain measurement set MR_Set2. For example, MR_Set1 may contain PRBs{0, 2, 6, 7, 8, 9, 10, 11} and MR_Set2 may contain PRBs {7, 8, 9, 10,11, 13, 15, 17}, where PRBs {7, 8, 9, 10, 11} are common to the twosets.

In yet another embodiment, MR_Set1 may contain all the PRBs in thesystem bandwidth and be applied to a first set of ports in the CSI-RSresource set CSI-RS_Resource_Set1. The second measurement restrictionset, MR_Set2, may contain a subset of PRBs and be applied to a secondset of ports in the CSI-RS resource set CSI-RS_Resource_Set2. An exampleis shown in FIG. 15 .

FIG. 15 illustrates an example of CSI-RS transmission where one set ofports are transmitted on all PRBs and another set of ports aretransmitted on a restricted set of PRBs, in accordance with certainembodiments. The resource element grid illustrated in FIG. 15 includes aportion of a subframe 10 with two PRBs 16. CSI-RS ports 15-22 aretransmitted in both PRBs 16, while CSI-RS ports 23-38 are transmitted ineven PRB 16.

In some embodiments, the eNB RRC configures the UE with K sets offrequency domain measurement restriction parameters MR_Setk, with k∈{0,1, . . . K−1} that applies to a k^(th) set of ports in the k^(th) CSI-RSresource set CSI-RS_Resource_Setk. The new CSI-RS reference number ofEquation 10 is modified as:i″=Kq′+i′  Equation 14Furthermore, the RE to port mapping formula in Equation 13 is modifiedas follows, where a new CSI-RS resource number i″ in the k^(th) CSI-RSresource set maps to a PRB index mm∈MR_Setk if i″∈CSI-RS_Resource_  Setk Equation 15

In some embodiments, CSI-RS port sets are RRC configured to the UEinstead of CSI-RS resource sets. For example, ifCSI-RS_port_set1={15-30} and CSI-RS_port_set2={31-46}, then MR_Set1 isapplied to ports {15-30} and MR_Set2 is applied to ports {31-46}.

A third group of example embodiments includes multiple ZP CSI-RSsubframe configurations. As shown in Table 4 above, ZP CSI-RS for aserving cell can only be configured with a single CSI-RS-SubframeConfigparameter I_(CSI-RS). This means that a UE can be configured with onlyone ZP CSI-RS in one subframe within the UE's configured ZP CSI-RSperiod T_(CSI-RS).

However, with the increased number of CSI-RS ports in LTE Release 14,achieving higher reuse factors wherein SINR can be improved through ZPCSI-RS than 1 for CSI-RS within a single subframe is not possible. Thisis because a given PRB within a subframe only includes 40 availableCSI-RS REs. If 32 of the REs are used for NZP CSI-RS by one cell, onlyone cell can transmit NZP CSI-RS in a subframe. Thus, to facilitatehigher reuse factors wherein SINR can be improved through ZP CSI-RS, inthis embodiment, a UE may be RRC configured with one NZP CSI-RS in a CSIprocess and ZP CSI-RSs that occur in multiple subframes within one ZPCSI-RS period in the CSI process. This may comprise configuring a UEwith a first and a second zero power CSI-RS occurring in a first and asecond subframe, wherein at least one of the first and second CSI-RS hasa periodicity of P subframes, and wherein the first and second subframesare distinct within the period P.

FIG. 16 illustrates an example of configuring multiple ZP CSI-RSsubframes, in accordance with certain embodiments. More particularly,FIG. 16 illustrates an example of three cells with CSI-RS configured indifferent subframes.

For each cell, two ZP CSI-RS are also configured in the subframes andREs where CSI-RS of the other two cells are configured. For example, ZPCSI-RS for cell1 is configured in subframes n+i, n+k, n+i+P and n+k+P,which coincide with the CSI-RS of cells 2 and 3. The first ZP CSI-RS forcell1 is configured in subframes n+i, n+i+P, . . . and the second ZPCSI-RS is configured in subframes n+k,n+k+P, . . . . In this case, theperiodicities of the two ZP CSI-RS are the same. When differentperiodicities are configured for CSI-RS in the three cells, then P mayalso be different for ZP CSI-RS.

Particular embodiments include RRC configuring the UE with multiple ZPCSI-RS subframe configurations. In a further embodiment, the UE is RRCconfigured with multiple configuration pairs, where each pair comprisesa ZP CSI-RS resource configuration that corresponds to a ZP CSI-RSsubframe configuration in the pair. Through these embodiments, thenetwork can configure a UE with ZP CSI-RS in multiple subframes wherethe ZP CSI-RS occupies the same REs as NZP CSI-RS intended for otherUEs. In this way, an eNB transmitting to a UE need not transmit PDSCH oran interfering NZP CSI-RS in the NZP CSI-RS intended for other UEs,which avoids interfering with the NZP CSI-RS intended for the other UEs.This will facilitate multiple serving cells to transmit NZP CSI-RS witha large number of ports (i.e., 32 ports) while avoiding interfering withneighboring cells' NZP CSI-RS.

The examples described with respect to FIGS. 12-16 may be generallyrepresented by the flowcharts in FIG. 17 (with respect to a networknode) and FIG. 18 (with respect to a wireless device).

FIG. 17 is a flow diagram illustrating an example method in a networknode of transmitting CSI-RS, according to some embodiments. Inparticular embodiments, one or more steps of FIG. 17 may be performed bynetwork node 120 of wireless network 100 described with respect to FIG.12 .

The method begins at step 1712, where a network node may obtain anindication of a subset of PRBs that a wireless device should use tomeasure CSI-RS. For example network node 120 may obtain an indication ofa subset of PRBs that wireless device 110 may use to measure CSI-RS.

Obtaining the indication may include retrieving a predeterminedindication from memory, receiving signaling from another component ofnetwork 100, or any other suitable configuration. The particularindication may be determined based on factors such as deploymentscenarios and load conditions.

At step 1714, the network node transmits the indication of the subset ofPRBs that the wireless device should use to measure CSI-RS to thewireless device. For example, network node 120 may transmit theindication of the subset of PRBs that wireless device 110 should use tomeasure CSI-RS to wireless device 110. The transmitting may include RRCsignaling, or any other suitable communication between network node 120and wireless device 100.

The indication of the subset of PRBs may comprise various formats, suchas the formats described above with respect to FIGS. 13-16 . Forexample, the format may include a bitmap, an odd/even value, a densityand comb offset, an index identifying a particular format of a group offormats known to the wireless device, etc. In some embodiments, theindication may include a number of subframes that wireless device 110should use to measure CSI-RS. At step 1716, the network node transmitsCSI-RS on the indicated subset of PRBs. For example, network node 120may transmit CSI-RS in the PRBs that network node 120 previouslyindicated to wireless device 110.

At step 1718, the network node may receive, from the wireless device, ameasured channel state information based on one or more of thetransmitted CSI-RS ports. For example, wireless device 110 may determinea CSI based on measurements of one or more of the transmitted CSI-RS andsend the CSI back to network node 120.

Modifications, additions, or omissions may be made to method 1700.Additionally, one or more steps in method 1700 of FIG. 17 may beperformed in parallel or in any suitable order. The steps of method 1700may be repeated over time as necessary.

FIG. 18 is a flow diagram illustrating an example method in a wirelessdevice of receiving CSI-RS, according to some embodiments. In particularembodiments, one or more steps of FIG. 18 may be performed by wirelessdevice 110 of wireless network 100 described with respect to FIG. 12 .

The method begins at step 1812, where the wireless device receives anindication of a subset of PRBs that the wireless device should use tomeasure CSI-RS. For example, wireless device 110 may receive, fromnetwork node 120, an indication of a subset of PRBs that wireless device110 should use to measure CSI-RS. The indication may comprise any of theindications described above with respect FIGS. 13-16 (e.g., theindication transmitted at step 1714 of FIG. 17 ).

At step 1814, the wireless device receives CSI-RS on the indicatedsubset of PRBs. For example, wireless device 110 may receive CSI-RS onthe indicated PRBs 16 from network node 120.

At step 1816, the wireless device may determine a CSI based on thereceived CSI-RS. For example, wireless device 110 may use the receivedindication to measure CSI-RS on the indicated PRBs 16 to estimate aneffective channel between network node 120 and wireless device 110. Insome embodiments, wireless device 110 may measure the CSI-RS overmultiple subframes.

At step 1818, the wireless device may transmit the CSI to a networknode. For example, wireless device 110 may transmit the CSI to networknode 120.

Modifications, additions, or omissions may be made to method 1800.Additionally, one or more steps in method 1800 of FIG. 18 may beperformed in parallel or in any suitable order. The steps of method 1800may be repeated over time as necessary.

FIG. 19A is a block diagram illustrating an example embodiment of awireless device. The wireless device is an example of the wirelessdevices 110 illustrated in FIG. 12 . In particular embodiments, thewireless device is capable of receiving an indication of a subset ofPRBs that the wireless device should use to measure CSI-RS, andreceiving CSI-RS on the indicated subset of PRBs. In particularembodiments, the wireless device may be capable of measuring thereceived CSI-RS ports to estimate an effective channel and determine aCSI, and transmitting the CSI to a network node.

Particular examples of a wireless device include a mobile phone, a smartphone, a PDA (Personal Digital Assistant), a portable computer (e.g.,laptop, tablet), a sensor, a modem, a machine type (MTC) device/machineto machine (M2M) device, laptop embedded equipment (LEE), laptop mountedequipment (LME), USB dongles, a device-to-device capable device, avehicle-to-vehicle device, or any other device that can provide wirelesscommunication. The wireless device includes processing circuitry 1900.Processing circuitry 1900 includes transceiver 1910, processor 1920,memory 1930, and power source 1940. In some embodiments, transceiver1910 facilitates transmitting wireless signals to and receiving wirelesssignals from wireless network node 120 (e.g., via an antenna), processor1920 executes instructions to provide some or all of the functionalitydescribed herein as provided by the wireless device, and memory 1930stores the instructions executed by processor 1920. Power source 1940supplies electrical power to one or more of the components of wirelessdevice 110, such as transceiver 1910, processor 1920, and/or memory1930.

Processor 1920 includes any suitable combination of hardware andsoftware implemented in one or more integrated circuits or modules toexecute instructions and manipulate data to perform some or all of thedescribed functions of the wireless device. In some embodiments,processor 1920 may include, for example, one or more computers, one moreprogrammable logic devices, one or more central processing units (CPUs),one or more microprocessors, one or more applications, and/or otherlogic, and/or any suitable combination of the preceding. Processor 1920may include analog and/or digital circuitry configured to perform someor all of the described functions of wireless device 110. For example,processor 1920 may include resistors, capacitors, inductors,transistors, diodes, and/or any other suitable circuit components.

Memory 1930 is generally operable to store computer executable code anddata. Examples of memory 1930 include computer memory (e.g., RandomAccess Memory (RAM) or Read Only Memory (ROM)), mass storage media(e.g., a hard disk), removable storage media (e.g., a Compact Disk (CD)or a Digital Video Disk (DVD)), and/or or any other volatile ornon-volatile, non-transitory computer-readable and/orcomputer-executable memory devices that store information.

Power source 1940 is generally operable to supply electrical power tothe components of wireless device 110. Power source 1940 may include anysuitable type of battery, such as lithium-ion, lithium-air, lithiumpolymer, nickel cadmium, nickel metal hydride, or any other suitabletype of battery for supplying power to a wireless device.

In particular embodiments, processor 1920 in communication withtransceiver 1910 receives an indication of a subset of PRBs that thewireless device should use to measure CSI-RS, and receives CSI-RS on theindicated subset of PRBs. In particular embodiments, processor 1920 incommunication with transceiver 1910 may measure the received CSI-RS toestimate an effective channel, and transmit a CSI to a network node.

Other embodiments of the wireless device may include additionalcomponents (beyond those shown in FIG. 19A) responsible for providingcertain aspects of the wireless device's functionality, including any ofthe functionality described above and/or any additional functionality(including any functionality necessary to support the solution describedabove).

FIG. 19B is a block diagram illustrating example components of awireless device 110. The components may include receiving module 1950,measuring module 1952, and transmitting module 1954.

Receiving module 1950 may perform the receiving functions of wirelessdevice 110. For example, receiving module 1950 may receive an indicationof a subset of PRBs that the wireless device should use to measureCSI-RS, and receive CSI-RS on the indicated subset of PRBs according toany of the examples described with respect to FIGS. 12-18 . In certainembodiments, receiving module 1950 may include or be included inprocessor 1920. In particular embodiments, receiving module 1950 maycommunicate with measuring module 1952 and transmitting module 1954.

Measuring module 1952 may perform the measuring functions of wirelessdevice 110. For example, measuring module 1952 may estimate a wirelesschannel using the received CSI-RS. Measuring module 1952 may determine aCSI based on the estimation. In certain embodiments, measuring module1952 may include or be included in processor 1920. In particularembodiments, measuring module 1952 may communicate with receiving module1950 and transmitting module 1954.

Transmitting module 1954 may perform the transmitting functions ofwireless device 110. For example, transmitting module 1954 may transmita CSI to network node 120. In certain embodiments, transmitting module1954 may include or be included in processor 1920. In particularembodiments, transmitting module 1954 may communicate with receivingmodule 1950 and measuring module 1952.

FIG. 20A is a block diagram illustrating an example embodiment of anetwork node. The network node is an example of the network node 120illustrated in FIG. 12 . In particular embodiments, the network node iscapable of: obtaining an indication of a subset of physical resourceblocks (PRBs) that a wireless device should use to measure CSI-RS;transmitting, to the wireless device, the indication of the subset ofPRBs that the wireless device should use to measure CSI-RS; andtransmitting CSI-RS on the indicated subset of PRBs.

Network node 120 can be an eNodeB, a nodeB, a base station, a wirelessaccess point (e.g., a Wi-Fi access point), a low power node, a basetransceiver station (BTS), a transmission point or node, a remote RFunit (RRU), a remote radio head (RRH), or other radio access node. Thenetwork node includes processing circuitry 2000. Processing circuitry2000 includes at least one transceiver 2010, at least one processor2020, at least one memory 2030, and at least one network interface 2040.Transceiver 2010 facilitates transmitting wireless signals to andreceiving wireless signals from a wireless device, such as wirelessdevices 110 (e.g., via an antenna); processor 2020 executes instructionsto provide some or all of the functionality described above as beingprovided by a network node 120; memory 2030 stores the instructionsexecuted by processor 2020; and network interface 2040 communicatessignals to backend network components, such as a gateway, switch,router, Internet, Public Switched Telephone Network (PSTN), controller,and/or other network nodes 120. Processor 2020 and memory 2030 can be ofthe same types as described with respect to processor 1920 and memory1930 of FIG. 19A above.

In some embodiments, network interface 2040 is communicatively coupledto processor 2020 and refers to any suitable device operable to receiveinput for network node 120, send output from network node 120, performsuitable processing of the input or output or both, communicate to otherdevices, or any combination of the preceding. Network interface 2040includes appropriate hardware (e.g., port, modem, network interfacecard, etc.) and software, including protocol conversion and dataprocessing capabilities, to communicate through a network.

In particular embodiments, processor 2020 in communication withtransceiver 2010 is operable to: obtain an indication of a subset ofPRBs that a wireless device should use to measure CSI-RS; transmit, tothe wireless device, the indication of the subset of PRBs that thewireless device should use to measure CSI-RS; and transmit CSI-RS on theindicated subset of PRBs.

Other embodiments of network node 120 include additional components(beyond those shown in FIG. 20A) responsible for providing certainaspects of the network node's functionality, including any of thefunctionality described above and/or any additional functionality(including any functionality necessary to support the solution describedabove). The various different types of network nodes may includecomponents having the same physical hardware but configured (e.g., viaprogramming) to support different radio access technologies, or mayrepresent partly or entirely different physical components.

FIG. 20B is a block diagram illustrating example components of a networknode 120. The components may include obtaining module 2050, transmittingmodule 2052, and receiving module 2054.

Obtaining module 2050 may perform the obtaining functions of networknode 120. For example, obtaining module 2050 may obtain an indication ofa subset of PRBs that a wireless device should use to measure CSI-RSaccording to any of the examples described with respect to FIGS. 12-18 .In certain embodiments, obtaining module 2050 may include or be includedin processor 2020. In particular embodiments, obtaining module 2050 maycommunicate with transmitting module 2052 and receiving module 2054.

Transmitting module 2052 may perform the transmitting functions ofnetwork node 120. For example, transmitting module 2052 may transmit anindication of the subset of PRBs that a wireless device should use tomeasure CSI-RS, and transmit CSI-RS on one or more PRBs according to anyof the examples described with respect to FIGS. 12-18 . In certainembodiments, transmitting module 2052 may include or be included inprocessor 2020. In particular embodiments, transmitting module 2052 maycommunicate with obtaining module 2050 and receiving module 2054.

Receiving module 2054 may perform the receiving functions of networknode 120. For example, receiving module 2054 may receive a CSI fromwireless device 110. In certain embodiments, receiving module 2054 mayinclude or be included in processor 1920. In particular embodiments,receiving module 2054 may communicate with obtaining module 2050 andtransmitting module 2052.

Modifications, additions, or omissions may be made to the systems andapparatuses disclosed herein without departing from the scope of theinvention. The components of the systems and apparatuses may beintegrated or separated. Moreover, the operations of the systems andapparatuses may be performed by more, fewer, or other components.Additionally, operations of the systems and apparatuses may be performedusing any suitable logic comprising software, hardware, and/or otherlogic. As used in this document, “each” refers to each member of a setor each member of a subset of a set.

Modifications, additions, or omissions may be made to the methodsdisclosed herein without departing from the scope of the invention. Themethods may include more, fewer, or other steps. Additionally, steps maybe performed in any suitable order.

Although this disclosure has been described in terms of certainembodiments, alterations and permutations of the embodiments will beapparent to those skilled in the art. Accordingly, the above descriptionof the embodiments does not constrain this disclosure. Other changes,substitutions, and alterations are possible without departing from thescope of this disclosure, as defined by the claims below.

Particular embodiments may be implemented within the framework of aparticular communication standard. The following examples provide anon-limiting example of how the proposed solutions could be implementedwithin the framework of a 3GPP TSG RAN standard. The changes describedare merely intended to illustrate how certain aspects of the proposedsolutions could be implemented in a particular standard. However, theproposed solutions could also be implemented in other suitable manners,both in the 3GPP Specification and in other specifications or standards.

For example, particular standards may include the following agreementswith regards to CSI-RS design for Class A eFD-MIMO. For {20, 24, 28, 32}ports, a CSI-RS resource for class A CSI reporting may comprise anaggregation of K CSI-RS configurations [i.e. RE patterns]. The number ofREs in the k^(th) configuration N_(k)∈{4, 8}. The same N_(k)=N may beused for all k. The following are not precluded: (a) per-port CSI-RSdensity per PRB=1; (b) different per-port CSI-RS densities for differentCSI-RS ports.

Particular examples include TDM. A UE may measure and report one set ofports in one subframe and a remaining set of ports are measured andreported in another subframe. A challenge with this scheme is how theeNB combines the CSI reports measured on different sets of CSI-RS portson different subframes. Furthermore, if the CSI corresponding todifferent sets of CSI-RS ports are measured/reported on different CSI-RSsubframes, the reported CSI may be adversely affected by frequencydrift/Doppler over the subframes. Simulation results with 24 portsindicate that the TDM based scheme can suffer between 10-20% linkthroughput loss compared to the FDM scheme in the SNR range 0-10 dB.

With regards to the overhead of the TDM scheme, the TDM scheme does notreduce CSI-RS overhead over one CSI-RS transmission periodicity. This isillustrated by a comparison between the TDM scheme and a CSI-RS schemewhere all ports are measured in a single subframe in FIGS. 21A and B.

In FIG. 21A, a CSI-RS design based on the TDM scheme for 32 ports isdepicted where CSI ports 15-30 are measured in subframe (k+1) and CSIports 31-46 are measured in subframe (k+2). Assuming a system with 2 CRSports, 3 OFDM symbols for PDCCH, 2 DMRS ports, and CSI-RS periodicityNp=5 ms, then the CSI-RS overhead of the TDM scheme over one CSI-RSperiod is approximately 6%.

Under the same assumption, the scheme in FIG. 21B where all 32 ports aremeasured in subframe (k+1) achieves the same CSI-RS overhead of 6%. Fromthis comparison, it is evident that the TDM scheme does not reduce theCSI-RS overhead and merely distributes the overhead over differentsubframes.

In some embodiments, the TDM scheme can be down-selected since it doesnot reduce CSI-RS overhead and merely distributes the CSI-RS overheadover different subframes.

Particular examples include FDM. A UE may be configured to measurechannels on a subset of CSI-RS ports on one fixed set of PRBs andanother subset of antenna ports on a different fixed set of PRBs. A32-port example is shown in FIG. 14 . In this example, CSI-RS ports15-30 are transmitted in even PRBs and CSI-RS ports 31-46 aretransmitted in odd PRBs.

Evaluating the performance of the FDM scheme may include system levelsimulations using a 32 port 8×4 dual polarized array with 2×1 subarrayvirtualization. The performance of a FDM scheme with a CSI-RS density of0.5 RE/RB/port may be compared to that of CSI-RS design with fulldensity (i.e., 1 RE/RB/port). The results for the 3D-UMi and 3D-UMascenarios are given in Table 5 and Table 6, respectively. In theseresults, the 32 port CSI-RS resource is attained by aggregating four8-port CSI-RS configurations with CDM-4 and 3 dB power boosting.

TABLE 5 Performance comparison in 3D-UMi Reference RU [%] 5 20 50Reference offered traffic [bps/Hz/cell] 0.20 0.63 1.15 FDM Full CSI-RSFDM Full CSI-RS FDM Full CSI-RS Scheme Density Scheme Density SchemeDensity Cell edge 2.00 1.99 1.09 1.14 0.36 0.48 througput [bps/Hz/user]Mean throughput 4.44 4.34 3.53 3.51 2.13 2.33 [bps/Hz/user] Cell edge 10 −4 0 −25 0 gain [%] Mean 2 0 1 0 −9 0 throughput gain [%]

TABLE 6 Performance comparison in 3D-UMa Reference RU [%] 5 20 50Reference offered traffic [bps/Hz/cell] 0.19 0.55 1.91 Full Full FullFDM CSI-RS FDM CSI-RS FDM CSI-RS Scheme Density Scheme Density SchemeDensity Cell edge througput 1.80 1.83 0.93 1.02 0.19 0.39 [bps/Hz/user]Mean throughput 4.26 4.18 3.27 3.25 1.56 2.09 [bps/Hz/user] Cell edgegain [%] −2 0 −9 0 −51 0 Mean throughput 2 0 1 0 −25 0 gain [%]Simulation parameters Carrier frequency  2 GHz Bandwidth 10 MHzScenarios 3D UMi 200 m ISD, 3D UMa 500 m ISD Antenna 8 × 4 with 2 × 1virtualization Configurations tilt: 130° for 3D-UMi and 122° for 3D-UMaWrapping Radio distance based UE receiver MMSE-IRC CSI periodicity  5 msCSI delay  5 ms CSI mode PUSCH Mode 3-2 Outer loop Link Yes, 10% BLERtarget Adaptation UE noise figure  9 dB eNB Tx power 41 dBm (3D-UMi), 46dBm (3D-UMa) Traffic model FTP Model 1, 500 kB packet size UE speed  3km/h Scheduling Proportional fair in time and frequency CRS interferenceNot modeled. Overhead accounted for 2 CRS ports. DMRS overhead  2 DMRSports CSI-RS Overhead accounted for. Channel estimation error modeled.Reuse factor 1 assumed. Codebook 2D Grid of Beams based on DFT HARQ Max5 retransmissions Antenna spacing 0.8 lambda in vertical, 0.5 lambda inhorizontal Handover margin  3 dB

These results show that the FDM scheme attains a small mean throughputgain of 2% at a very low resource utilization of 5% due to the loweroverhead advantage it has over the full density CSI-RS scheme. However,at higher resource utilizations, the FDM scheme suffers notablethroughput losses. At 50% RU, the cell edge performance of the FDMscheme is 25% (in 3D-UMi) and 53% (in 3D-UMa) lower than the fulldensity CSI-RS scheme. This loss is mainly due to the reduced processinggain associated with FDM scheme when compared to the full density CSI-RSscheme.

FDM scheme with 0.5 RE/RB/port attains small gains at very small loadsbut suffers significant losses at medium to high loads when compared toa CSI-RS design with a density of 1 RE/RB/port. Thus, given the resultsin Table 5 and Table 6, FDM based designs with fixed CSI-RS densitiesmay not be a good solution due to its poor performance at high loads. Toensure good performance at medium to high load conditions, sufficientconfigurability in the CSI-RS design should be allowed to also haveper-port CSI-RS densities of 1 RE/RB/port in addition to CSI-RSdensities lower than 1 RE/RB/port.

FDM based CSI-RS designs with fixed CSI-RS densities may not beconsidered for Class A eFD-MIMO. Sufficient configurability in theCSI-RS design should be allowed to also have per-port CSI-RS densitiesof 1 RE/RB/port in addition to CSI-RS densities lower than 1 RE/RB/port.

Particular embodiments include measurement restriction in the frequencydomain. Because specifying enhancements of {20, 24, 28, 32} CSI-RS portswith mechanism for reducing the overhead for CSI-RS transmission is oneof the objectives of an eFD-MIMO standard, a more flexible approach isto allow the density of the CSI-RS design to be configurable. This canbe achieved via measurement restriction (MR) in frequency domain where aUE can be requested to measure channels on a configurable set of PRBs.CSI-RS is only transmitted in PRBs in which the UE is requested toperform channel measurements. The MR in frequency domain may besemi-statically configured and may be RRC signaled to the UE.

Because the MR in frequency domain is configurable, the density of theCSI-RS port can be flexibly chosen to suit the deployment scenario. Forinstance, for low-load, low-delay spread conditions, CSI-RS ports can beconfigured with reduced density. For high-load and/or high delay spreadconditions, a higher density can be configured for CSI-RS ports to avoidthe performance losses demonstrated in the results of Table 5 and Table6.

Several alternatives for reducing per-port CSI-RS density can beachieved via MR in frequency domain. A few examples include thefollowing.

FDM: With MR in frequency domain, FDM schemes with different CSI-RSdensities can be achieved. For instance, the 32 port reduced densityCSI-RS example of FIG. 14 can be achieved by configuring the UE tomeasure CSI-RS ports 15-30 on PRBs 0, 2, 4, 6, . . . and to measureCSI-RS ports 31-46 on PRBs 1, 3, 5, 7, . . . . Other density reductionfactors (i.e., 3 or 4) can also be configured with MR in frequencydomain if such reduction factor are suitable for a given deploymentscenario.

Partial Overlapping: Partially overlapping CSI-RS designs can beattained with MR in frequency domain. Considering the 32-port examplegiven in FIG. 15 , the UE is configured to measure CSI-RS ports 23-38only on PRBs 1, 3, 5, 7, . . . . For CSI-RS ports 15-22, the UE isconfigured to measure CSI-RS on all PRBs.

Partial bandwidth measurements: MR in frequency domain can beeffectively used to probe the UE to measure CSI-RS only on one or moresubbands in the context of aperiodic CSI-RS.

Full CSI-RS density: A per-port CSI-RS density of 1 RE/port/PRB can beachieved via MR in frequency domain by configuring the UE to measureCSI-RS on all PRBs.

MR in frequency domain can be applied for cases with different CSI-RSresources with different number of ports and cases involving differentCDM designs.

Given these advantages, particular embodiments include measurementrestriction in frequency domain used to achieve many of the alternativesfor reducing per-port CSI-RS density including FDM, partial overlapping,partial bandwidth, and full CSI-RS density. For Class A eFD-MIMO,measurement restriction in the frequency domain provides goodflexibility to configure the density of CSI-RS according to thedeployment scenario and load conditions.

Some embodiments include CSI-RS SINR improvements. The performance of a32-port CSI-RS design with CDM-4 and 3 dB power boosting (9 dB gain) maybe compared to the upper bound performance of a 32-port CSI-RS designwith 15 dB gain. Both cases assume a CSI-RS density of 1 RE/RB/port anda 32 port 8×4 dual polarized array with 2×1 subarray virtualization.Detailed simulation parameters are given above.

The results for the 3D-UMi and 3D-UMa scenarios at 50% resourceutilization are given in Table 7. These results show cell-edgethroughput upper bound gains of 29-41% when using the 15 dB gain whencompared to the case with 9 dB gain. The corresponding mean throughputgains are in the range of 11-12%. This suggests that further gains arepossible if the CSI-RS SINR can be further improved.

TABLE 7 Performance comparison between 9 dB and 15 dB gain cases at 50%reference RU Scenario 3D-UMi 3D-UMa Reference offered traffic[bps/Hz/cell] 1.15 0.91 Performance Upper Performance Upper with 9 dBBound with with 9 dB Bound with Gain 15 dB Gain Gain 15 dB Gain Celledge 0.48 0.62 0.39 0.55 throughput [bps/Hz/user] Mean 2.33 2.58 2.092.35 throughput [bps/Hz/user] Cell edge 0 29 0 41 gain [%] Mean 0 11 012 throughput gain [%]

Abbreviations used in the preceding description include:

3GPP Third Generation Partnership Project

AP Access Point

BSC Base Station Controller

BTS Base Transceiver Station

CDM Code Division Multiplexing

CPE Customer Premises Equipment

CRS Cell Specific Reference Signal

CQI Channel Quality Indicator

CSI Channel State Information

CSI-RS Channel State Information Reference Signal

D2D Device to Device

DAS Distributed Antenna System

DCI Downlink Control Information

DFT Discrete Fourier Transform

DL Downlink

DMRS Demodulation Reference Signal

eNB eNodeB

EPDCCH Enhanced Physical Downlink Control Channel

FDD Frequency Division Duplex

LTE Long Term Evolution

LAN Local Area Network

LEE Laptop Embedded Equipment

LME Laptop Mounted Equipment

MAC Medium Access Control

M2M Machine to Machine

MIMO Multi-Input Multi-Output

MR Measurement Restriction

MTC Machine Type Communication

NR New Radio

NZP Non-Zero Power

OCC Orthogonal Cover Code

OFDM Orthogonal Frequency Division Multiplexing

PDCCH Physical Downlink Control Channel

PDSCH Physical Downlink Shared Channel

PMI Precoded Matrix Indicator

PRB Physical Resource Block

PSTN Public Switched Telephone Network

PUSCH Physical Uplink Shared Channel

PUCCH Physical Uplink Control Channel

RAN Radio Access Network

RAT Radio Access Technology

RB Resource Block

RBS Radio Base Station

RE Resource Element

RI Rank Indicator

RNC Radio Network Controller

RRC Radio Resource Control

RRH Remote Radio Head

RRU Remote Radio Unit

TDD Time Division Duplex

TFRE Time Frequency Resource Element

TM Transmission Mode

UE User Equipment

UL Uplink

UTRAN Universal Terrestrial Radio Access Network

WAN Wireless Access Network

ZP Zero Power

The invention claimed is:
 1. A method for use in a network node oftransmitting channel state information reference signals (CSI-RS), themethod comprising: transmitting, to a wireless device, an indication ofa subset of physical resource blocks (PRBs) that are available for thewireless device to measure CSI-RS, each CSI-RS associated with anantenna port, either the subset of PRBs comprising even numbered PRBs orthe subset of PRBs comprising odd numbered PRBs; and transmitting CSI-RSon the indicated subset of PRBs, wherein the network node transmitsCSI-RS on a total number of antenna ports, and each PRB of the subset ofPRBs includes a CSI-RS mapping for the total number of antenna ports. 2.The method of claim 1, further comprising obtaining the indication ofthe subset of PRBs that are available for the wireless device to measureCSI-RS.
 3. The method of claim 1, wherein the indication of the subsetof PRBs that are available for the wireless device to measure CSI-RSports comprises a density value and a comb offset.
 4. The method ofclaim 3, wherein: the density value comprises a density of ½; a firstcomb offset indicates that the wireless device should use the PRBs in aset m1 to measure CSI-RS, wherein the set m1 comprises {0, 2, . . . ,N_(RB) ^(DL)−2}; and a second comb offset indicates that the wirelessdevice should use the PRBs in a set m2 to measure CSI-RS, wherein theset m2 comprises {1, 3, . . . , N_(RB) ^(DL)−1}.
 5. The method of claim3, wherein: the density value comprises a density of ⅓; a first comboffset indicates that the wireless device should use the PRBs in a setm1 to measure CSI-RS, wherein the set m1 comprises {0, 3, . . . , N_(RB)^(DL)−3}; a second comb offset indicates that the wireless device shoulduse the PRBs in a set m2 to measure CSI-RS, wherein the set m2 comprises{1, 4, . . . , N_(RB) ^(DL)−2}; and a third comb offset indicates thatthe wireless device should use the PRBs in a set m3 to measure CSI-RS,wherein the set m3 comprises {2, 5, . . . , N_(RB) ^(DL)−1}.
 6. Themethod of claim 1, wherein the indication of the subset of PRBs that areavailable for the wireless device to measure CSI-RS comprises an indexvalue k, the index value k referring to one of a plurality ofindications stored at the wireless device.
 7. The method of claim 1,wherein the indication of the subset of PRBs that are available for thewireless device to measure CSI-RS further comprises a number ofsuccessive CSI-RS subframes in which the wireless device should measureCSI-RS.
 8. A method for use in a wireless device of receiving channelstate information reference signals (CSI-RS), the method comprising:receiving an indication of a subset of physical resource blocks (PRBs)that are available for the wireless device to measure CSI-RS, each ofthe CSI-RS associated with an antenna port, either the subset of PRBscomprising even numbered PRBs or the subset of PRBs comprising oddnumbered PRBs; and receiving CSI-RS on the indicated subset of PRBs,wherein the wireless device receives CSI-RS transmitted on a totalnumber of antenna ports from a network node, and each PRB of the subsetof PRBs includes a CSI-RS mapping for the total number of antenna ports.9. The method of claim 8, further comprising: determining a channelstate information (CSI) based on the received CSI-RS; and transmittingthe CSI to a network node.
 10. The method of claim 8, wherein theindication of the subset of PRBs that are available for the wirelessdevice to measure CSI-RS comprises a density value and a comb offset.11. The method of claim 10, wherein: the density value comprises adensity of ½; a first comb offset indicates that the wireless deviceshould use the PRBs in a set m1 to measure CSI-RS, wherein the set m1comprises {0, 2, . . . , N_(RB) ^(DL)−2}; and a second comb offsetindicates that the wireless device should use the PRBs in a set m2 tomeasure CSI-RS, wherein the set m2 comprises {1, 3, . . . , N_(RB)^(DL)−1}.
 12. The method of claim 10, wherein: the density valuecomprises a density of ⅓; a first comb offset indicates that thewireless device should use the PRBs in a set m1 to measure CSI-RS,wherein the set m1 comprises {0, 3, . . . , N_(RB) ^(DL)−3}; a secondcomb offset indicates that the wireless device should use the PRBs in aset m2 to measure CSI-RS, wherein the set m2 comprises {1, 4, . . . ,N_(RB) ^(DL)−2}; and a third comb offset indicates that the wirelessdevice should use the PRBs in a set m3 to measure CSI-RS, wherein theset m3 comprises {2, 5, . . . , N_(RB) ^(DL)−1}.
 13. The method of claim8, wherein the indication of the subset of PRBs that are available forthe wireless device to measure CSI-RS comprises an index value k, theindex value k referring to one of a plurality of indications stored atthe wireless device.
 14. The method of claim 8, wherein the indicationof the subset of PRBs that are available for the wireless device tomeasure CSI-RS further comprises a number of successive CSI-RS subframesin which the wireless device should measure CSI-RS.
 15. The method ofclaim 14, further comprising determining a channel state information(CSI) based on the received CSI-RS over the number of successive CSI-RSsubframes.
 16. A network node operable to transmit channel stateinformation reference signals (CSI-RS) (135), the network nodecomprising processing circuitry, the processing circuitry operable to:transmit, to a wireless device, an indication of a subset of physicalresource blocks (PRBs) that are available for the wireless device tomeasure CSI-RS, each CSI-RS associated with an antenna port, either thesubset of PRBs comprising even numbered PRBs or the subset of PRBscomprising odd numbered PRBs; and transmit CSI-RS on the indicatedsubset of PRBs, wherein the network node transmits CSI-RS on a totalnumber of antenna ports, and each PRB of the subset of PRBs includes aCSI-RS mapping for the total number of antenna ports.
 17. The networknode of claim 16, the processing circuitry further operable to obtainthe indication of the subset of PRBs that are available for the wirelessdevice to measure CSI-RS.
 18. The network node of claim 16, wherein theindication of the subset of PRBs that are available for the wirelessdevice to measure CSI-RS comprises a density value and a comb offset.19. The network node of claim 16, wherein the indication of the subsetof PRBs that are available for the wireless device measure CSI-RScomprises an index value k, the index value k referring to one of aplurality of indications stored at the wireless device.
 20. The networknode of claim 16, wherein the indication of the subset of PRBs that areavailable for the wireless device to measure CSI-RS further comprises anumber of successive CSI-RS subframes in which the wireless deviceshould measure CSI-RS.
 21. A wireless device operable to receive channelstate information reference signals (CSI-RS), the wireless devicecomprising processing circuitry, the processing circuitry operable to:receive an indication of a subset of physical resource blocks (PRBs)that are available for the wireless device to measure CSI-RS, each ofthe CSI-RS associated with an antenna port, either the subset of PRBscomprising even numbered PRBs or the subset of PRBs comprising oddnumbered PRBs; and receive CSI-RS on the indicated subset of PRBs,wherein the wireless device receives CSI-RS transmitted on a totalnumber of antenna ports from a network node, and each PRB of the subsetof PRBs includes a CSI-RS mapping for the total number of antenna ports.22. The wireless device of claim 21, wherein the indication of thesubset of PRBs that are available for the wireless device to measureCSI-RS ports comprises a density value and a comb offset.
 23. Thewireless device of claim 21, wherein the indication of the subset ofPRBs that are available for the wireless device to measure CSI-RScomprises an index value k, the index value k referring to one of aplurality of indications stored at the wireless device.
 24. The wirelessdevice of claim 21, wherein the indication of the subset of PRBs thatare available for the wireless device to measure CSI-RS furthercomprises a number of successive CSI-RS subframes that the wirelessdevice should measure CSI-RS.
 25. The wireless device of claim 24, theprocessing circuitry further operable to determine a channel stateinformation (CSI) based on the received CSI-RS over the number ofsuccessive CSI-RS subframes.